[PATCH v3 0/3] Fix attaching to process when it has zombie threads

[PATCH v3 0/3] Fix attaching to process when it has zombie threads

[Thiago Jung Bauermann]

Hello,

This version just adjusts the comment in patch 1 as suggested by Pedro.
This reflects in patch 2, which has a slightly abbreviated version of the
comment.

v2 of this series is here:

https://inbox.sourceware.org/gdb-patches/20240420055652.819024-1-thiago.bauermann@linaro.org/

v1 of this series is here:

https://inbox.sourceware.org/gdb-patches/20240321231149.519549-1-thiago.bauermann@linaro.org/


Thiago Jung Bauermann (3):
  gdb/nat: Use procfs(5) indexes in linux_common_core_of_thread
  gdb/nat: Factor linux_proc_get_stat_field out of
    linux_common_core_of_thread
  gdb/nat/linux: Fix attaching to process when it has zombie threads

 gdb/nat/linux-osdata.c | 38 +++--------------
 gdb/nat/linux-osdata.h |  3 ++
 gdb/nat/linux-procfs.c | 97 ++++++++++++++++++++++++++++++++++++++++++
 gdb/nat/linux-procfs.h | 11 +++++
 4 files changed, 116 insertions(+), 33 deletions(-)


base-commit: f5ef12c3f1af890dcdc6dc54f5ea58a05c6cf65a

[PATCH v3 1/3] gdb/nat: Use procfs(5) indexes in linux_common_core_of_thread

[Thiago Jung Bauermann]

The code and comment reference stat fields by made-up indexes.  The
procfs(5) man page, which describes the /proc/PID/stat file, has a numbered
list of these fields so it's more convenient to use those numbers instead.

This is currently an implementation detail inside the function so it's
not really relevant with the code as-is, but a future patch will do some
refactoring which will make the index more prominent.

Therefore, make this change in a separate patch so that it's simpler to
review.

Reviewed-By: Luis Machado <luis.machado@arm.com>
---
 gdb/nat/linux-osdata.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

Changes in v3:
- Use macro names in comment, and clarify why we use those fields
  (suggested by Pedro).

Changes in v2:
- Added macros for field indexes in /proc/PID/stat (Suggested by Luis).

diff --git a/gdb/nat/linux-osdata.c b/gdb/nat/linux-osdata.c
index 6ffabe90aa7d..4812bc735e86 100644
--- a/gdb/nat/linux-osdata.c
+++ b/gdb/nat/linux-osdata.c
@@ -52,6 +52,10 @@ typedef long long TIME_T;
 
 #define MAX_PID_T_STRLEN  (sizeof ("-9223372036854775808") - 1)
 
+/* Index of fields of interest in /proc/PID/stat, from procfs(5) man page.  */
+#define LINUX_PROC_STAT_STATE 3
+#define LINUX_PROC_STAT_PROCESSOR 39
+
 /* Returns the CPU core that thread PTID is currently running on.  */
 
 /* Compute and return the processor core of a given thread.  */
@@ -74,10 +78,9 @@ linux_common_core_of_thread (ptid_t ptid)
   if (pos == std::string::npos)
     return -1;
 
-  /* If the first field after program name has index 0, then core number is
-     the field with index 36 (so, the 37th).  There's no constant for that
-     anywhere.  */
-  for (int i = 0; i < 37; ++i)
+  /* The first field after program name is LINUX_PROC_STAT_STATE, and we are
+     interested in field LINUX_PROC_STAT_PROCESSOR.  */
+  for (int i = LINUX_PROC_STAT_STATE; i <= LINUX_PROC_STAT_PROCESSOR; ++i)
     {
       /* Find separator.  */
       pos = content->find_first_of (' ', pos);

[PATCH v3 2/3] gdb/nat: Factor linux_proc_get_stat_field out of linux_common_core_of_thread

[Thiago Jung Bauermann]

The new function will be used in a subsequent patch to read a different
stat field.

The new code is believed to be equivalent to the old code, so there
should be no change in GDB behaviour.  The only material change was to
use std::string and string_printf rather than a fixed char array to
build the path to the stat file.

Also, take the opportunity to move the function's documentation comment to
the header file, to conform with GDB practice.

Reviewed-By: Luis Machado <luis.machado@arm.com>
---
 gdb/nat/linux-osdata.c | 41 +++++----------------------------------
 gdb/nat/linux-osdata.h |  3 +++
 gdb/nat/linux-procfs.c | 44 ++++++++++++++++++++++++++++++++++++++++++
 gdb/nat/linux-procfs.h | 10 ++++++++++
 4 files changed, 62 insertions(+), 36 deletions(-)

Changes in v3:
- Preserve comment mentioning why we use that field in the for loop in
  linux_proc_get_stat_field.

Changes in v2:
- Added macros for field indexes in /proc/PID/stat (Suggested by Luis).
- Moved linux_find_proc_stat_field from linux-osdata.c to linux-procfs.c
  and changed prefix to linux_proc (Suggested by Pedro).
- Use string_printf in linux_proc_get_stat_field to build path to stat
  file, to avoid having to copy the PID_T and MAX_PID_T_STRLEN macros.

diff --git a/gdb/nat/linux-osdata.c b/gdb/nat/linux-osdata.c
index 4812bc735e86..3a6215015f12 100644
--- a/gdb/nat/linux-osdata.c
+++ b/gdb/nat/linux-osdata.c
@@ -36,6 +36,7 @@
 #include <sys/stat.h>
 #include "gdbsupport/filestuff.h"
 #include <algorithm>
+#include "linux-procfs.h"
 
 #define NAMELEN(dirent) strlen ((dirent)->d_name)
 
@@ -52,50 +53,18 @@ typedef long long TIME_T;
 
 #define MAX_PID_T_STRLEN  (sizeof ("-9223372036854775808") - 1)
 
-/* Index of fields of interest in /proc/PID/stat, from procfs(5) man page.  */
-#define LINUX_PROC_STAT_STATE 3
-#define LINUX_PROC_STAT_PROCESSOR 39
-
-/* Returns the CPU core that thread PTID is currently running on.  */
-
-/* Compute and return the processor core of a given thread.  */
+/* See linux-osdata.h.  */
 
 int
 linux_common_core_of_thread (ptid_t ptid)
 {
-  char filename[sizeof ("/proc//task//stat") + 2 * MAX_PID_T_STRLEN];
+  std::optional<std::string> field
+    = linux_proc_get_stat_field (ptid, LINUX_PROC_STAT_PROCESSOR);
   int core;
 
-  sprintf (filename, "/proc/%lld/task/%lld/stat",
-	   (PID_T) ptid.pid (), (PID_T) ptid.lwp ());
-
-  std::optional<std::string> content = read_text_file_to_string (filename);
-  if (!content.has_value ())
+  if (!field.has_value () || sscanf (field->c_str (), "%d", &core) == 0)
     return -1;
 
-  /* ps command also relies on no trailing fields ever contain ')'.  */
-  std::string::size_type pos = content->find_last_of (')');
-  if (pos == std::string::npos)
-    return -1;
-
-  /* The first field after program name is LINUX_PROC_STAT_STATE, and we are
-     interested in field LINUX_PROC_STAT_PROCESSOR.  */
-  for (int i = LINUX_PROC_STAT_STATE; i <= LINUX_PROC_STAT_PROCESSOR; ++i)
-    {
-      /* Find separator.  */
-      pos = content->find_first_of (' ', pos);
-      if (pos == std::string::npos)
-	return {};
-
-      /* Find beginning of field.  */
-      pos = content->find_first_not_of (' ', pos);
-      if (pos == std::string::npos)
-	return {};
-    }
-
-  if (sscanf (&(*content)[pos], "%d", &core) == 0)
-    core = -1;
-
   return core;
 }
 
diff --git a/gdb/nat/linux-osdata.h b/gdb/nat/linux-osdata.h
index 833915cdb2fd..a82fb08b998e 100644
--- a/gdb/nat/linux-osdata.h
+++ b/gdb/nat/linux-osdata.h
@@ -20,7 +20,10 @@
 #ifndef NAT_LINUX_OSDATA_H
 #define NAT_LINUX_OSDATA_H
 
+/* Returns the CPU core that thread PTID is currently running on.  */
+
 extern int linux_common_core_of_thread (ptid_t ptid);
+
 extern LONGEST linux_common_xfer_osdata (const char *annex, gdb_byte *readbuf,
 					 ULONGEST offset, ULONGEST len);
 
diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
index e2086952ce6b..c11eaf3cc6fd 100644
--- a/gdb/nat/linux-procfs.c
+++ b/gdb/nat/linux-procfs.c
@@ -230,6 +230,50 @@ linux_proc_pid_is_zombie (pid_t pid)
 
 /* See linux-procfs.h.  */
 
+std::optional<std::string>
+linux_proc_get_stat_field (ptid_t ptid, int field)
+{
+  /* We never need to read PID from the stat file, and there's
+     command_from_pid to read the comm field.  */
+  gdb_assert (field >= LINUX_PROC_STAT_STATE);
+
+  std::string filename = string_printf ("/proc/%ld/task/%ld/stat",
+					(long) ptid.pid (), (long) ptid.lwp ());
+
+  std::optional<std::string> content
+    = read_text_file_to_string (filename.c_str ());
+  if (!content.has_value ())
+    return {};
+
+  /* ps command also relies on no trailing fields ever contain ')'.  */
+  std::string::size_type pos = content->find_last_of (')');
+  if (pos == std::string::npos)
+    return {};
+
+  /* The first field after program name is LINUX_PROC_STAT_STATE.  */
+  for (int i = LINUX_PROC_STAT_STATE; i <= field; ++i)
+    {
+      /* Find separator.  */
+      pos = content->find_first_of (' ', pos);
+      if (pos == std::string::npos)
+	return {};
+
+      /* Find beginning of field.  */
+      pos = content->find_first_not_of (' ', pos);
+      if (pos == std::string::npos)
+	return {};
+    }
+
+  /* Find end of field.  */
+  std::string::size_type end_pos = content->find_first_of (' ', pos);
+  if (end_pos == std::string::npos)
+    return content->substr (pos);
+  else
+    return content->substr (pos, end_pos - pos);
+}
+
+/* See linux-procfs.h.  */
+
 const char *
 linux_proc_tid_get_name (ptid_t ptid)
 {
diff --git a/gdb/nat/linux-procfs.h b/gdb/nat/linux-procfs.h
index 880afbcdd615..ec1f37651fbf 100644
--- a/gdb/nat/linux-procfs.h
+++ b/gdb/nat/linux-procfs.h
@@ -54,6 +54,16 @@ extern int linux_proc_pid_is_zombie_nowarn (pid_t pid);
 
 extern int linux_proc_pid_is_gone (pid_t pid);
 
+/* Index of fields of interest in /proc/PID/stat, from procfs(5) man page.  */
+#define LINUX_PROC_STAT_STATE 3
+#define LINUX_PROC_STAT_PROCESSOR 39
+
+/* Returns FIELD (as numbered in procfs(5) man page) of
+   /proc/PID/task/LWP/stat file.  */
+
+extern std::optional<std::string> linux_proc_get_stat_field (ptid_t ptid,
+							     int field);
+
 /* Return a string giving the thread's name or NULL if the
    information is unavailable.  The returned value points to a statically
    allocated buffer.  The value therefore becomes invalid at the next

Re: [PATCH v3 2/3] gdb/nat: Factor linux_proc_get_stat_field out of linux_common_core_of_thread

[Luis Machado]

On 4/25/24 00:15, Thiago Jung Bauermann wrote:
> The new function will be used in a subsequent patch to read a different
> stat field.
> 
> The new code is believed to be equivalent to the old code, so there
> should be no change in GDB behaviour.  The only material change was to
> use std::string and string_printf rather than a fixed char array to
> build the path to the stat file.
> 
> Also, take the opportunity to move the function's documentation comment to
> the header file, to conform with GDB practice.
> 
> Reviewed-By: Luis Machado <luis.machado@arm.com>
> ---
>  gdb/nat/linux-osdata.c | 41 +++++----------------------------------
>  gdb/nat/linux-osdata.h |  3 +++
>  gdb/nat/linux-procfs.c | 44 ++++++++++++++++++++++++++++++++++++++++++
>  gdb/nat/linux-procfs.h | 10 ++++++++++
>  4 files changed, 62 insertions(+), 36 deletions(-)
> 
> Changes in v3:
> - Preserve comment mentioning why we use that field in the for loop in
>   linux_proc_get_stat_field.
> 
> Changes in v2:
> - Added macros for field indexes in /proc/PID/stat (Suggested by Luis).
> - Moved linux_find_proc_stat_field from linux-osdata.c to linux-procfs.c
>   and changed prefix to linux_proc (Suggested by Pedro).
> - Use string_printf in linux_proc_get_stat_field to build path to stat
>   file, to avoid having to copy the PID_T and MAX_PID_T_STRLEN macros.
> 
> diff --git a/gdb/nat/linux-osdata.c b/gdb/nat/linux-osdata.c
> index 4812bc735e86..3a6215015f12 100644
> --- a/gdb/nat/linux-osdata.c
> +++ b/gdb/nat/linux-osdata.c
> @@ -36,6 +36,7 @@
>  #include <sys/stat.h>
>  #include "gdbsupport/filestuff.h"
>  #include <algorithm>
> +#include "linux-procfs.h"
>  
>  #define NAMELEN(dirent) strlen ((dirent)->d_name)
>  
> @@ -52,50 +53,18 @@ typedef long long TIME_T;
>  
>  #define MAX_PID_T_STRLEN  (sizeof ("-9223372036854775808") - 1)
>  
> -/* Index of fields of interest in /proc/PID/stat, from procfs(5) man page.  */
> -#define LINUX_PROC_STAT_STATE 3
> -#define LINUX_PROC_STAT_PROCESSOR 39
> -
> -/* Returns the CPU core that thread PTID is currently running on.  */
> -
> -/* Compute and return the processor core of a given thread.  */
> +/* See linux-osdata.h.  */
>  
>  int
>  linux_common_core_of_thread (ptid_t ptid)
>  {
> -  char filename[sizeof ("/proc//task//stat") + 2 * MAX_PID_T_STRLEN];
> +  std::optional<std::string> field
> +    = linux_proc_get_stat_field (ptid, LINUX_PROC_STAT_PROCESSOR);
>    int core;
>  
> -  sprintf (filename, "/proc/%lld/task/%lld/stat",
> -	   (PID_T) ptid.pid (), (PID_T) ptid.lwp ());
> -
> -  std::optional<std::string> content = read_text_file_to_string (filename);
> -  if (!content.has_value ())
> +  if (!field.has_value () || sscanf (field->c_str (), "%d", &core) == 0)
>      return -1;
>  
> -  /* ps command also relies on no trailing fields ever contain ')'.  */
> -  std::string::size_type pos = content->find_last_of (')');
> -  if (pos == std::string::npos)
> -    return -1;
> -
> -  /* The first field after program name is LINUX_PROC_STAT_STATE, and we are
> -     interested in field LINUX_PROC_STAT_PROCESSOR.  */
> -  for (int i = LINUX_PROC_STAT_STATE; i <= LINUX_PROC_STAT_PROCESSOR; ++i)
> -    {
> -      /* Find separator.  */
> -      pos = content->find_first_of (' ', pos);
> -      if (pos == std::string::npos)
> -	return {};
> -
> -      /* Find beginning of field.  */
> -      pos = content->find_first_not_of (' ', pos);
> -      if (pos == std::string::npos)
> -	return {};
> -    }
> -
> -  if (sscanf (&(*content)[pos], "%d", &core) == 0)
> -    core = -1;
> -
>    return core;
>  }
>  
> diff --git a/gdb/nat/linux-osdata.h b/gdb/nat/linux-osdata.h
> index 833915cdb2fd..a82fb08b998e 100644
> --- a/gdb/nat/linux-osdata.h
> +++ b/gdb/nat/linux-osdata.h
> @@ -20,7 +20,10 @@
>  #ifndef NAT_LINUX_OSDATA_H
>  #define NAT_LINUX_OSDATA_H
>  
> +/* Returns the CPU core that thread PTID is currently running on.  */
> +
>  extern int linux_common_core_of_thread (ptid_t ptid);
> +
>  extern LONGEST linux_common_xfer_osdata (const char *annex, gdb_byte *readbuf,
>  					 ULONGEST offset, ULONGEST len);
>  
> diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
> index e2086952ce6b..c11eaf3cc6fd 100644
> --- a/gdb/nat/linux-procfs.c
> +++ b/gdb/nat/linux-procfs.c
> @@ -230,6 +230,50 @@ linux_proc_pid_is_zombie (pid_t pid)
>  
>  /* See linux-procfs.h.  */
>  
> +std::optional<std::string>
> +linux_proc_get_stat_field (ptid_t ptid, int field)
> +{
> +  /* We never need to read PID from the stat file, and there's
> +     command_from_pid to read the comm field.  */
> +  gdb_assert (field >= LINUX_PROC_STAT_STATE);
> +
> +  std::string filename = string_printf ("/proc/%ld/task/%ld/stat",
> +					(long) ptid.pid (), (long) ptid.lwp ());
> +
> +  std::optional<std::string> content
> +    = read_text_file_to_string (filename.c_str ());
> +  if (!content.has_value ())
> +    return {};
> +
> +  /* ps command also relies on no trailing fields ever contain ')'.  */

Probably an existing typo, but s/contain/containing?

No need to send a v5 for this though.

> +  std::string::size_type pos = content->find_last_of (')');
> +  if (pos == std::string::npos)
> +    return {};
> +
> +  /* The first field after program name is LINUX_PROC_STAT_STATE.  */
> +  for (int i = LINUX_PROC_STAT_STATE; i <= field; ++i)
> +    {
> +      /* Find separator.  */
> +      pos = content->find_first_of (' ', pos);
> +      if (pos == std::string::npos)
> +	return {};
> +
> +      /* Find beginning of field.  */
> +      pos = content->find_first_not_of (' ', pos);
> +      if (pos == std::string::npos)
> +	return {};
> +    }
> +
> +  /* Find end of field.  */
> +  std::string::size_type end_pos = content->find_first_of (' ', pos);
> +  if (end_pos == std::string::npos)
> +    return content->substr (pos);
> +  else
> +    return content->substr (pos, end_pos - pos);
> +}
> +
> +/* See linux-procfs.h.  */
> +
>  const char *
>  linux_proc_tid_get_name (ptid_t ptid)
>  {
> diff --git a/gdb/nat/linux-procfs.h b/gdb/nat/linux-procfs.h
> index 880afbcdd615..ec1f37651fbf 100644
> --- a/gdb/nat/linux-procfs.h
> +++ b/gdb/nat/linux-procfs.h
> @@ -54,6 +54,16 @@ extern int linux_proc_pid_is_zombie_nowarn (pid_t pid);
>  
>  extern int linux_proc_pid_is_gone (pid_t pid);
>  
> +/* Index of fields of interest in /proc/PID/stat, from procfs(5) man page.  */
> +#define LINUX_PROC_STAT_STATE 3
> +#define LINUX_PROC_STAT_PROCESSOR 39
> +
> +/* Returns FIELD (as numbered in procfs(5) man page) of
> +   /proc/PID/task/LWP/stat file.  */
> +
> +extern std::optional<std::string> linux_proc_get_stat_field (ptid_t ptid,
> +							     int field);
> +
>  /* Return a string giving the thread's name or NULL if the
>     information is unavailable.  The returned value points to a statically
>     allocated buffer.  The value therefore becomes invalid at the next

Otherwise looks OK.

Re: [PATCH v3 2/3] gdb/nat: Factor linux_proc_get_stat_field out of linux_common_core_of_thread

[Luis Machado]

On 4/25/24 09:59, Luis Machado wrote:
> On 4/25/24 00:15, Thiago Jung Bauermann wrote:
>> The new function will be used in a subsequent patch to read a different
>> stat field.
>>
>> The new code is believed to be equivalent to the old code, so there
>> should be no change in GDB behaviour.  The only material change was to
>> use std::string and string_printf rather than a fixed char array to
>> build the path to the stat file.
>>
>> Also, take the opportunity to move the function's documentation comment to
>> the header file, to conform with GDB practice.
>>
>> Reviewed-By: Luis Machado <luis.machado@arm.com>
>> ---
>>  gdb/nat/linux-osdata.c | 41 +++++----------------------------------
>>  gdb/nat/linux-osdata.h |  3 +++
>>  gdb/nat/linux-procfs.c | 44 ++++++++++++++++++++++++++++++++++++++++++
>>  gdb/nat/linux-procfs.h | 10 ++++++++++
>>  4 files changed, 62 insertions(+), 36 deletions(-)
>>
>> Changes in v3:
>> - Preserve comment mentioning why we use that field in the for loop in
>>   linux_proc_get_stat_field.
>>
>> Changes in v2:
>> - Added macros for field indexes in /proc/PID/stat (Suggested by Luis).
>> - Moved linux_find_proc_stat_field from linux-osdata.c to linux-procfs.c
>>   and changed prefix to linux_proc (Suggested by Pedro).
>> - Use string_printf in linux_proc_get_stat_field to build path to stat
>>   file, to avoid having to copy the PID_T and MAX_PID_T_STRLEN macros.
>>
>> diff --git a/gdb/nat/linux-osdata.c b/gdb/nat/linux-osdata.c
>> index 4812bc735e86..3a6215015f12 100644
>> --- a/gdb/nat/linux-osdata.c
>> +++ b/gdb/nat/linux-osdata.c
>> @@ -36,6 +36,7 @@
>>  #include <sys/stat.h>
>>  #include "gdbsupport/filestuff.h"
>>  #include <algorithm>
>> +#include "linux-procfs.h"
>>  
>>  #define NAMELEN(dirent) strlen ((dirent)->d_name)
>>  
>> @@ -52,50 +53,18 @@ typedef long long TIME_T;
>>  
>>  #define MAX_PID_T_STRLEN  (sizeof ("-9223372036854775808") - 1)
>>  
>> -/* Index of fields of interest in /proc/PID/stat, from procfs(5) man page.  */
>> -#define LINUX_PROC_STAT_STATE 3
>> -#define LINUX_PROC_STAT_PROCESSOR 39
>> -
>> -/* Returns the CPU core that thread PTID is currently running on.  */
>> -
>> -/* Compute and return the processor core of a given thread.  */
>> +/* See linux-osdata.h.  */
>>  
>>  int
>>  linux_common_core_of_thread (ptid_t ptid)
>>  {
>> -  char filename[sizeof ("/proc//task//stat") + 2 * MAX_PID_T_STRLEN];
>> +  std::optional<std::string> field
>> +    = linux_proc_get_stat_field (ptid, LINUX_PROC_STAT_PROCESSOR);
>>    int core;
>>  
>> -  sprintf (filename, "/proc/%lld/task/%lld/stat",
>> -	   (PID_T) ptid.pid (), (PID_T) ptid.lwp ());
>> -
>> -  std::optional<std::string> content = read_text_file_to_string (filename);
>> -  if (!content.has_value ())
>> +  if (!field.has_value () || sscanf (field->c_str (), "%d", &core) == 0)
>>      return -1;
>>  
>> -  /* ps command also relies on no trailing fields ever contain ')'.  */
>> -  std::string::size_type pos = content->find_last_of (')');
>> -  if (pos == std::string::npos)
>> -    return -1;
>> -
>> -  /* The first field after program name is LINUX_PROC_STAT_STATE, and we are
>> -     interested in field LINUX_PROC_STAT_PROCESSOR.  */
>> -  for (int i = LINUX_PROC_STAT_STATE; i <= LINUX_PROC_STAT_PROCESSOR; ++i)
>> -    {
>> -      /* Find separator.  */
>> -      pos = content->find_first_of (' ', pos);
>> -      if (pos == std::string::npos)
>> -	return {};
>> -
>> -      /* Find beginning of field.  */
>> -      pos = content->find_first_not_of (' ', pos);
>> -      if (pos == std::string::npos)
>> -	return {};
>> -    }
>> -
>> -  if (sscanf (&(*content)[pos], "%d", &core) == 0)
>> -    core = -1;
>> -
>>    return core;
>>  }
>>  
>> diff --git a/gdb/nat/linux-osdata.h b/gdb/nat/linux-osdata.h
>> index 833915cdb2fd..a82fb08b998e 100644
>> --- a/gdb/nat/linux-osdata.h
>> +++ b/gdb/nat/linux-osdata.h
>> @@ -20,7 +20,10 @@
>>  #ifndef NAT_LINUX_OSDATA_H
>>  #define NAT_LINUX_OSDATA_H
>>  
>> +/* Returns the CPU core that thread PTID is currently running on.  */
>> +
>>  extern int linux_common_core_of_thread (ptid_t ptid);
>> +
>>  extern LONGEST linux_common_xfer_osdata (const char *annex, gdb_byte *readbuf,
>>  					 ULONGEST offset, ULONGEST len);
>>  
>> diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
>> index e2086952ce6b..c11eaf3cc6fd 100644
>> --- a/gdb/nat/linux-procfs.c
>> +++ b/gdb/nat/linux-procfs.c
>> @@ -230,6 +230,50 @@ linux_proc_pid_is_zombie (pid_t pid)
>>  
>>  /* See linux-procfs.h.  */
>>  
>> +std::optional<std::string>
>> +linux_proc_get_stat_field (ptid_t ptid, int field)
>> +{
>> +  /* We never need to read PID from the stat file, and there's
>> +     command_from_pid to read the comm field.  */
>> +  gdb_assert (field >= LINUX_PROC_STAT_STATE);
>> +
>> +  std::string filename = string_printf ("/proc/%ld/task/%ld/stat",
>> +					(long) ptid.pid (), (long) ptid.lwp ());
>> +
>> +  std::optional<std::string> content
>> +    = read_text_file_to_string (filename.c_str ());
>> +  if (!content.has_value ())
>> +    return {};
>> +
>> +  /* ps command also relies on no trailing fields ever contain ')'.  */
> 
> Probably an existing typo, but s/contain/containing?
> 
> No need to send a v5 for this though.
> 

Eh, or v4 rather. :-)

>> +  std::string::size_type pos = content->find_last_of (')');
>> +  if (pos == std::string::npos)
>> +    return {};
>> +
>> +  /* The first field after program name is LINUX_PROC_STAT_STATE.  */
>> +  for (int i = LINUX_PROC_STAT_STATE; i <= field; ++i)
>> +    {
>> +      /* Find separator.  */
>> +      pos = content->find_first_of (' ', pos);
>> +      if (pos == std::string::npos)
>> +	return {};
>> +
>> +      /* Find beginning of field.  */
>> +      pos = content->find_first_not_of (' ', pos);
>> +      if (pos == std::string::npos)
>> +	return {};
>> +    }
>> +
>> +  /* Find end of field.  */
>> +  std::string::size_type end_pos = content->find_first_of (' ', pos);
>> +  if (end_pos == std::string::npos)
>> +    return content->substr (pos);
>> +  else
>> +    return content->substr (pos, end_pos - pos);
>> +}
>> +
>> +/* See linux-procfs.h.  */
>> +
>>  const char *
>>  linux_proc_tid_get_name (ptid_t ptid)
>>  {
>> diff --git a/gdb/nat/linux-procfs.h b/gdb/nat/linux-procfs.h
>> index 880afbcdd615..ec1f37651fbf 100644
>> --- a/gdb/nat/linux-procfs.h
>> +++ b/gdb/nat/linux-procfs.h
>> @@ -54,6 +54,16 @@ extern int linux_proc_pid_is_zombie_nowarn (pid_t pid);
>>  
>>  extern int linux_proc_pid_is_gone (pid_t pid);
>>  
>> +/* Index of fields of interest in /proc/PID/stat, from procfs(5) man page.  */
>> +#define LINUX_PROC_STAT_STATE 3
>> +#define LINUX_PROC_STAT_PROCESSOR 39
>> +
>> +/* Returns FIELD (as numbered in procfs(5) man page) of
>> +   /proc/PID/task/LWP/stat file.  */
>> +
>> +extern std::optional<std::string> linux_proc_get_stat_field (ptid_t ptid,
>> +							     int field);
>> +
>>  /* Return a string giving the thread's name or NULL if the
>>     information is unavailable.  The returned value points to a statically
>>     allocated buffer.  The value therefore becomes invalid at the next
> 
> Otherwise looks OK.

Re: [PATCH v3 2/3] gdb/nat: Factor linux_proc_get_stat_field out of linux_common_core_of_thread

[Thiago Jung Bauermann]

Hello Luis,

Luis Machado <luis.machado@arm.com> writes:

> On 4/25/24 09:59, Luis Machado wrote:
>> On 4/25/24 00:15, Thiago Jung Bauermann wrote:
>>> diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
>>> index e2086952ce6b..c11eaf3cc6fd 100644
>>> --- a/gdb/nat/linux-procfs.c
>>> +++ b/gdb/nat/linux-procfs.c
>>> @@ -230,6 +230,50 @@ linux_proc_pid_is_zombie (pid_t pid)
>>>  
>>>  /* See linux-procfs.h.  */
>>>  
>>> +std::optional<std::string>
>>> +linux_proc_get_stat_field (ptid_t ptid, int field)
>>> +{
>>> +  /* We never need to read PID from the stat file, and there's
>>> +     command_from_pid to read the comm field.  */
>>> +  gdb_assert (field >= LINUX_PROC_STAT_STATE);
>>> +
>>> +  std::string filename = string_printf ("/proc/%ld/task/%ld/stat",
>>> +					(long) ptid.pid (), (long) ptid.lwp ());
>>> +
>>> +  std::optional<std::string> content
>>> +    = read_text_file_to_string (filename.c_str ());
>>> +  if (!content.has_value ())
>>> +    return {};
>>> +
>>> +  /* ps command also relies on no trailing fields ever contain ')'.  */
>> 
>> Probably an existing typo, but s/contain/containing?
>> 
>> No need to send a v5 for this though.
>> 
>
> Eh, or v4 rather. :-)

I think you're right. Fixed locally.

>>> +  std::string::size_type pos = content->find_last_of (')');
>>> +  if (pos == std::string::npos)
>>> +    return {};
>>> +
>>> +  /* The first field after program name is LINUX_PROC_STAT_STATE.  */
>>> +  for (int i = LINUX_PROC_STAT_STATE; i <= field; ++i)
>>> +    {
>>> +      /* Find separator.  */
>>> +      pos = content->find_first_of (' ', pos);
>>> +      if (pos == std::string::npos)
>>> +	return {};
>>> +
>>> +      /* Find beginning of field.  */
>>> +      pos = content->find_first_not_of (' ', pos);
>>> +      if (pos == std::string::npos)
>>> +	return {};
>>> +    }
>>> +
>>> +  /* Find end of field.  */
>>> +  std::string::size_type end_pos = content->find_first_of (' ', pos);
>>> +  if (end_pos == std::string::npos)
>>> +    return content->substr (pos);
>>> +  else
>>> +    return content->substr (pos, end_pos - pos);
>>> +}

<snip>

>> Otherwise looks OK.

Thanks!

-- 
Thiago

[PATCH v3 3/3] gdb/nat/linux: Fix attaching to process when it has zombie threads

[Thiago Jung Bauermann]

When GDB attaches to a multi-threaded process, it calls
linux_proc_attach_tgid_threads () to go through all threads found in
/proc/PID/task/ and call attach_proc_task_lwp_callback () on each of
them.  If it does that twice without the callback reporting that a new
thread was found, then it considers that all inferior threads have been
found and returns.

The problem is that the callback considers any thread that it hasn't
attached to yet as new.  This causes problems if the process has one or
more zombie threads, because GDB can't attach to it and the loop will
always "find" a new thread (the zombie one), and get stuck in an
infinite loop.

This is easy to trigger (at least on aarch64-linux and powerpc64le-linux)
with the gdb.threads/attach-many-short-lived-threads.exp testcase, because
its test program constantly creates and finishes joinable threads so the
chance of having zombie threads is high.

This problem causes the following failures:

FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: attach (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: no new threads (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: set breakpoint always-inserted on (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: break break_fn (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: break at break_fn: 1 (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: break at break_fn: 2 (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: break at break_fn: 3 (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: reset timer in the inferior (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: print seconds_left (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: detach (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: set breakpoint always-inserted off (timeout)
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: delete all breakpoints, watchpoints, tracepoints, and catchpoints in delete_breakpoints (timeout)
ERROR: breakpoints not deleted

The iteration number is random, and all tests in the subsequent iterations
fail too, because GDB is stuck in the attach command at the beginning of
the iteration.

The solution is to make linux_proc_attach_tgid_threads () remember when it
has already processed a given LWP and skip it in the subsequent iterations.

PR testsuite/31312
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31312

Reviewed-By: Luis Machado <luis.machado@arm.com>
Approved-By: Pedro Alves <pedro@palves.net>
---
 gdb/nat/linux-procfs.c | 53 ++++++++++++++++++++++++++++++++++++++++++
 gdb/nat/linux-procfs.h |  1 +
 2 files changed, 54 insertions(+)

No change in v3.

Changes in v2:
- Added macro for field index in /proc/PID/stat (Suggested by Luis).
- Moved linux_get_starttime to linux-procfs.c and changed its prefix
  to linux_proc (Suggested by Pedro).
- Changed visited_lwps from std::set to std::unordered_set. Had to add a
  hash function (Suggested by Pedro).

diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
index c11eaf3cc6fd..ba6263d79e5f 100644
--- a/gdb/nat/linux-procfs.c
+++ b/gdb/nat/linux-procfs.c
@@ -20,6 +20,8 @@
 #include "gdbsupport/filestuff.h"
 #include <dirent.h>
 #include <sys/stat.h>
+#include <unordered_set>
+#include <utility>
 
 /* Return the TGID of LWPID from /proc/pid/status.  Returns -1 if not
    found.  */
@@ -272,6 +274,29 @@ linux_proc_get_stat_field (ptid_t ptid, int field)
     return content->substr (pos, end_pos - pos);
 }
 
+/* Get the start time of thread PTID.  */
+
+static std::optional<ULONGEST>
+linux_proc_get_starttime (ptid_t ptid)
+{
+  std::optional<std::string> field
+    = linux_proc_get_stat_field (ptid, LINUX_PROC_STAT_STARTTIME);
+
+  if (!field.has_value ())
+    return {};
+
+  errno = 0;
+  const char *trailer;
+  ULONGEST starttime = strtoulst (field->c_str (), &trailer, 10);
+  if (starttime == ULONGEST_MAX && errno == ERANGE)
+    return {};
+  else if (*trailer != '\0')
+    /* There were unexpected characters.  */
+    return {};
+
+  return starttime;
+}
+
 /* See linux-procfs.h.  */
 
 const char *
@@ -333,6 +358,21 @@ linux_proc_attach_tgid_threads (pid_t pid,
       return;
     }
 
+  /* Callable object to hash elements in visited_lpws.  */
+  struct pair_hash
+  {
+    std::size_t operator() (const std::pair<unsigned long, ULONGEST> &v) const
+    {
+      return (std::hash<unsigned long>() (v.first)
+	      ^ std::hash<ULONGEST>() (v.second));
+    }
+  };
+
+  /* Keeps track of the LWPs we have already visited in /proc,
+     identified by their PID and starttime to detect PID reuse.  */
+  std::unordered_set<std::pair<unsigned long, ULONGEST>,
+		     pair_hash> visited_lwps;
+
   /* Scan the task list for existing threads.  While we go through the
      threads, new threads may be spawned.  Cycle through the list of
      threads until we have done two iterations without finding new
@@ -351,6 +391,19 @@ linux_proc_attach_tgid_threads (pid_t pid,
 	  if (lwp != 0)
 	    {
 	      ptid_t ptid = ptid_t (pid, lwp);
+	      std::optional<ULONGEST> starttime
+		= linux_proc_get_starttime (ptid);
+
+	      if (starttime.has_value ())
+		{
+		  std::pair<unsigned long, ULONGEST> key (lwp, *starttime);
+
+		  /* If we already visited this LWP, skip it this time.  */
+		  if (visited_lwps.find (key) != visited_lwps.cend ())
+		    continue;
+
+		  visited_lwps.insert (key);
+		}
 
 	      if (attach_lwp (ptid))
 		new_threads_found = 1;
diff --git a/gdb/nat/linux-procfs.h b/gdb/nat/linux-procfs.h
index ec1f37651fbf..64224801c8f2 100644
--- a/gdb/nat/linux-procfs.h
+++ b/gdb/nat/linux-procfs.h
@@ -56,6 +56,7 @@ extern int linux_proc_pid_is_gone (pid_t pid);
 
 /* Index of fields of interest in /proc/PID/stat, from procfs(5) man page.  */
 #define LINUX_PROC_STAT_STATE 3
+#define LINUX_PROC_STAT_STARTTIME 22
 #define LINUX_PROC_STAT_PROCESSOR 39
 
 /* Returns FIELD (as numbered in procfs(5) man page) of

Re: [PATCH v3 0/3] Fix attaching to process when it has zombie threads

[Pedro Alves]

On 2024-04-25 00:15, Thiago Jung Bauermann wrote:
> Hello,
> 
> This version just adjusts the comment in patch 1 as suggested by Pedro.
> This reflects in patch 2, which has a slightly abbreviated version of the
> comment.
> 
> v2 of this series is here:
> 
> https://inbox.sourceware.org/gdb-patches/20240420055652.819024-1-thiago.bauermann@linaro.org/
> 
> v1 of this series is here:
> 
> https://inbox.sourceware.org/gdb-patches/20240321231149.519549-1-thiago.bauermann@linaro.org/
> 
> 
> Thiago Jung Bauermann (3):
>   gdb/nat: Use procfs(5) indexes in linux_common_core_of_thread
>   gdb/nat: Factor linux_proc_get_stat_field out of
>     linux_common_core_of_thread
>   gdb/nat/linux: Fix attaching to process when it has zombie threads
> 
>  gdb/nat/linux-osdata.c | 38 +++--------------
>  gdb/nat/linux-osdata.h |  3 ++
>  gdb/nat/linux-procfs.c | 97 ++++++++++++++++++++++++++++++++++++++++++
>  gdb/nat/linux-procfs.h | 11 +++++
>  4 files changed, 116 insertions(+), 33 deletions(-)
> 

For series:

  Approved-By: Pedro Alves <pedro@palves.net>

Re: [PATCH v3 0/3] Fix attaching to process when it has zombie threads

[Thiago Jung Bauermann]

Pedro Alves <pedro@palves.net> writes:

> On 2024-04-25 00:15, Thiago Jung Bauermann wrote:
>> Hello,
>> 
>> This version just adjusts the comment in patch 1 as suggested by Pedro.
>> This reflects in patch 2, which has a slightly abbreviated version of the
>> comment.
>> 
>> v2 of this series is here:
>> 
>> https://inbox.sourceware.org/gdb-patches/20240420055652.819024-1-thiago.bauermann@linaro.org/
>> 
>> v1 of this series is here:
>> 
>> https://inbox.sourceware.org/gdb-patches/20240321231149.519549-1-thiago.bauermann@linaro.org/
>> 
>> 
>> Thiago Jung Bauermann (3):
>>   gdb/nat: Use procfs(5) indexes in linux_common_core_of_thread
>>   gdb/nat: Factor linux_proc_get_stat_field out of
>>     linux_common_core_of_thread
>>   gdb/nat/linux: Fix attaching to process when it has zombie threads
>> 
>>  gdb/nat/linux-osdata.c | 38 +++--------------
>>  gdb/nat/linux-osdata.h |  3 ++
>>  gdb/nat/linux-procfs.c | 97 ++++++++++++++++++++++++++++++++++++++++++
>>  gdb/nat/linux-procfs.h | 11 +++++
>>  4 files changed, 116 insertions(+), 33 deletions(-)
>> 
>
> For series:
>
>   Approved-By: Pedro Alves <pedro@palves.net>

Thank you! Pushed as commits 3de4256ca3e8, 16a447bec542 and
c930a077225e.

-- 
Thiago