Re: [PATCH] linux_nat_target::xfer_partial: Fallback to ptrace

[Pedro Alves <pedro@palves.net>]

Hi Keith,

On 2022-07-21 4:03 p.m., Keith Seitz wrote:
> Ping

Appologies for the delay, there was something here that I wanted to think about, and then it
just fell through the cracks.

See below.

> On 6/3/22 08:18, Keith Seitz via Gdb-patches wrote:
>> Pedro Alves wrote:
>>
>>> I guess the write to /proc/pid/mem fails with EIO for you, and there's nothing
>>> else we can use to detect the scenario.  So we probably want to check TARGET_XFER_E_IO
>>> instead.  And, maybe only do the fallback if writing.
>>
>> I've updated the patch to include these changes and retested on all the "usual"
>> target boards on Fedora 36 x86_64, s390x, ppc64le, and aarch64 and on RHEL6
>> x86_64.

Thanks, that's a lot of testing.

>> Commit 05c06f318fd9a112529dfc313e6512b399a645e4 enabled GDB
>> to access memory while threads are running. It did this by accessing
>> /proc/PID/task/LWP/mem.
>>
>> Unfortunately, this interface is not implemented for writing in older kernels
>> (such as RHEL6). This means that GDB is unable to insert breakpoints on
>> these hosts:
>>
>> $ ./gdb -q gdb -ex start
>> Reading symbols from gdb...
>> Temporary breakpoint 1 at 0x40fdd5: file ../../src/gdb/gdb.c, line 28.
>> Starting program: /home/rhel6/fsf/linux/gdb/gdb
>> Warning:
>> Cannot insert breakpoint 1.
>> Cannot access memory at address 0x40fdd5
>>
>> (gdb)
>>
>> Before this patch, linux_proc_xfer_memory_partial (previously called
>> linux_proc_xfer_partial) would return TARGET_XFER_EOF if the write
>> to /proc/PID/mem failed. [More specifically, linux_proc_xfer_partial would
>> not "bother for one word," but the effect is the essentially same.]
>>
>> This status was checked by linux_nat_target::xfer_partial, which would then
>> fallback to using ptrace to perform the operation.
>>
>> This is the specific hunk that removed the fallback:
>>
>> -  xfer = linux_proc_xfer_partial (object, annex, readbuf, writebuf,
>> -                                 offset, len, xfered_len);
>> -  if (xfer != TARGET_XFER_EOF)
>> -    return xfer;
>> +      return linux_proc_xfer_memory_partial (readbuf, writebuf,
>> +                                            offset, len, xfered_len);
>> +    }
>>
>>     return inf_ptrace_target::xfer_partial (object, annex, readbuf, writebuf,
>>                                            offset, len, xfered_len);
>>
>> This patch restores this fallback mechanism, enabling GDB to insert
>> breakpoints on these older kernels. Note that a recent patch changed
>> the return status from TARGET_XFER_EOF to TARGET_XFER_E_IO.
>>
>> Tested on {unix,native-gdbserver,native-extended-gdbserver}/-m{32,64}
>> on x86_64, s390x, aarch64, and ppc64le.
>> ---
>>   gdb/linux-nat.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
>> index 3b5400896bc..571c97137c2 100644
>> --- a/gdb/linux-nat.c
>> +++ b/gdb/linux-nat.c
>> @@ -3706,8 +3706,12 @@ linux_nat_target::xfer_partial (enum target_object object,
>>         if (addr_bit < (sizeof (ULONGEST) * HOST_CHAR_BIT))
>>       offset &= ((ULONGEST) 1 << addr_bit) - 1;
>>   -      return linux_proc_xfer_memory_partial (readbuf, writebuf,
>> -                         offset, len, xfered_len);
>> +      enum target_xfer_status xfer
>> +    = linux_proc_xfer_memory_partial (readbuf, writebuf,
>> +                      offset, len, xfered_len);
>> +      if (xfer != TARGET_XFER_E_IO || readbuf != nullptr)
>> +    return xfer;
>> +      /* Fallthrough to ptrace.  /proc/pid/mem wasn't writable before Linux 2.6.39.  */
>>       }
>>       return inf_ptrace_target::xfer_partial (object, annex, readbuf, writebuf,
> 

So my worry here is this bringing back a race with more modern kernels, one which the
always writing via /proc/pid/mem plugged -- when the inferior execs, writes through /proc/pid/mem fail,
as the old pre-exec address space is gone.  Only when we see the exec even out of ptrace, will
we close the file and reopen a new one to access the post-exec address space.  Up until recent
kernels, ptrace calls didn't have a similar protection.  I.e., if the inferior execs, and the
write through /proc/pid/mem fails because of an exec, and then we fallback to ptrace, that write
will succeed, but it will be writing bytes in the post-exec address space that were meant
for the pre-exec address space.

So I was wondering about mitigating this by only falling back to ptrace if writing to
/proc/pid/mem doesn't really work.  Checking the kernel version itself seems a bit
fragile, so I thought we could make gdb probe once at started up whether writing
to itself via /proc/self/mem works.  It turns out that actually works.  With this, you'd
just add an extra proc_mem_file_is_writable() check in your patch before falling
back, or even, skip straight to ptrace if !proc_mem_file_is_writable().

WDYT?

From 56622b9cadff4b62a0b05861015ce06cf9d6e8f2 Mon Sep 17 00:00:00 2001
From: Pedro Alves <pedro@palves.net>
Date: Thu, 21 Jul 2022 19:11:16 +0100
Subject: [PATCH] gdb/linux-nat: Check whether /proc/pid/mem is writable

Probe whether /proc/pid/mem is writable, by using it to write to a GDB
variable.

Change-Id: If87eff0b46cbe5e32a583e2977a9e17d29d0ed3e
---
 gdb/linux-nat.c | 106 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 89 insertions(+), 17 deletions(-)

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 0a93ab5c6ae..0916b841ec7 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -3674,6 +3674,8 @@ static enum target_xfer_status
 linux_proc_xfer_memory_partial (gdb_byte *readbuf, const gdb_byte *writebuf,
 				ULONGEST offset, LONGEST len, ULONGEST *xfered_len);
 
+static bool proc_mem_file_is_writable ();
+
 enum target_xfer_status
 linux_nat_target::xfer_partial (enum target_object object,
 				const char *annex, gdb_byte *readbuf,
@@ -3882,25 +3884,19 @@ open_proc_mem_file (ptid_t ptid)
 			  fd, ptid.pid (), ptid.lwp ());
 }
 
-/* Implement the to_xfer_partial target method using /proc/PID/mem.
-   Because we can use a single read/write call, this can be much more
-   efficient than banging away at PTRACE_PEEKTEXT.  Also, unlike
-   PTRACE_PEEKTEXT/PTRACE_POKETEXT, this works with running
-   threads.  */
+/* Helper for linux_proc_xfer_memory_partial and
+   proc_mem_file_is_writable.  FD is the already opened /proc/pid/mem
+   file, and PID is the pid of the corresponding process.  The rest of
+   the arguments are like linux_proc_xfer_memory_partial's.  */
 
 static enum target_xfer_status
-linux_proc_xfer_memory_partial (gdb_byte *readbuf, const gdb_byte *writebuf,
-				ULONGEST offset, LONGEST len,
-				ULONGEST *xfered_len)
+linux_proc_xfer_memory_partial_fd (int fd, int pid,
+				   gdb_byte *readbuf, const gdb_byte *writebuf,
+				   ULONGEST offset, LONGEST len,
+				   ULONGEST *xfered_len)
 {
   ssize_t ret;
 
-  auto iter = proc_mem_file_map.find (inferior_ptid.pid ());
-  if (iter == proc_mem_file_map.end ())
-    return TARGET_XFER_EOF;
-
-  int fd = iter->second.fd ();
-
   gdb_assert (fd != -1);
 
   /* Use pread64/pwrite64 if available, since they save a syscall and can
@@ -3919,8 +3915,7 @@ linux_proc_xfer_memory_partial (gdb_byte *readbuf, const gdb_byte *writebuf,
   if (ret == -1)
     {
       linux_nat_debug_printf ("accessing fd %d for pid %d failed: %s (%d)",
-			      fd, inferior_ptid.pid (),
-			      safe_strerror (errno), errno);
+			      fd, pid, safe_strerror (errno), errno);
       return TARGET_XFER_E_IO;
     }
   else if (ret == 0)
@@ -3928,7 +3923,7 @@ linux_proc_xfer_memory_partial (gdb_byte *readbuf, const gdb_byte *writebuf,
       /* EOF means the address space is gone, the whole process exited
 	 or execed.  */
       linux_nat_debug_printf ("accessing fd %d for pid %d got EOF",
-			      fd, inferior_ptid.pid ());
+			      fd, pid);
       return TARGET_XFER_EOF;
     }
   else
@@ -3938,6 +3933,81 @@ linux_proc_xfer_memory_partial (gdb_byte *readbuf, const gdb_byte *writebuf,
     }
 }
 
+/* Implement the to_xfer_partial target method using /proc/PID/mem.
+   Because we can use a single read/write call, this can be much more
+   efficient than banging away at PTRACE_PEEKTEXT.  Also, unlike
+   PTRACE_PEEKTEXT/PTRACE_POKETEXT, this works with running
+   threads.  */
+
+static enum target_xfer_status
+linux_proc_xfer_memory_partial (gdb_byte *readbuf, const gdb_byte *writebuf,
+				ULONGEST offset, LONGEST len,
+				ULONGEST *xfered_len)
+{
+  int pid = inferior_ptid.pid ();
+
+  auto iter = proc_mem_file_map.find (pid);
+  if (iter == proc_mem_file_map.end ())
+    return TARGET_XFER_EOF;
+
+  int fd = iter->second.fd ();
+
+  return linux_proc_xfer_memory_partial_fd (fd, pid, readbuf, writebuf, offset,
+					    len, xfered_len);
+}
+
+/* Check whether /proc/pid/mem is writable in the current kernel, and
+   return true if so.  It wasn't writable before Linux 2.6.39, but
+   there's no way to know whether the feature was backported to older
+   kernels.  So we check to see if it works.  The result is cached,
+   and this is garanteed to be called once early at startup.  */
+
+static bool
+proc_mem_file_is_writable ()
+{
+  static gdb::optional<bool> writable;
+
+  if (writable.has_value ())
+    return *writable;
+
+  *writable = false;
+
+  /* We check whether /proc/pid/mem is writable by trying to write to
+     one of our variables via /proc/self/mem.  */
+
+  int fd = gdb_open_cloexec ("/proc/self/mem", O_RDWR | O_LARGEFILE, 0).release ();
+
+  if (fd == -1)
+    {
+      warning (_("opening /proc/self/mem file failed: %s (%d)"),
+	       safe_strerror (errno), errno);
+      return *writable;
+    }
+
+  SCOPE_EXIT { close (fd); };
+
+  /* This is the variable we try to write to.  Note OFFSET below.  */
+  volatile static gdb_byte test_var = 0;
+
+  gdb_byte writebuf[] = {0x55};
+  ULONGEST offset = (uintptr_t) &test_var;
+  ULONGEST xfered_len;
+
+  enum target_xfer_status res
+    = linux_proc_xfer_memory_partial_fd (fd, getpid (), nullptr, writebuf,
+					 offset, 1, &xfered_len);
+
+  if (res == TARGET_XFER_OK)
+    {
+      gdb_assert (xfered_len == 1);
+      gdb_assert (test_var == 0x55);
+      /* Success.  */
+      *writable = true;
+    }
+
+  return *writable;
+}
+
 /* Parse LINE as a signal set and add its set bits to SIGS.  */
 
 static void
@@ -4437,6 +4507,8 @@ Enables printf debugging output."),
   sigemptyset (&blocked_mask);
 
   lwp_lwpid_htab_create ();
+
+  proc_mem_file_is_writable ();
 }
 \f
 

base-commit: c07ec968f7342a2386969bc192ff0b42e33991ef
-- 
2.36.0