[binutils-gdb] gdb/nat/linux: Fix attaching to process when it has zombie threads

[binutils-gdb] gdb/nat/linux: Fix attaching to process when it has zombie threads

[Thiago Bauermann]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=c930a077225ec042287379d8e49b4d547f97d1ba

commit c930a077225ec042287379d8e49b4d547f97d1ba
Author: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Date:   Sun Mar 17 02:40:05 2024 -0300

    gdb/nat/linux: Fix attaching to process when it has zombie threads
    
    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>

Diff:
---
 gdb/nat/linux-procfs.c | 53 ++++++++++++++++++++++++++++++++++++++++++++++++++
 gdb/nat/linux-procfs.h |  1 +
 2 files changed, 54 insertions(+)

diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
index 081a82be050..5efc86cca1f 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 ec1f37651fb..64224801c8f 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