Re: [PATCH v1 2/2] gdb: rely on the first non-exited thread TPID when reading Linux procfs files

[Matthieu Longo <matthieu.longo@arm.com>]

On 03/08/2026 05:48, Thiago Jung Bauermann wrote:
> Hello Matthieu,
> 
> Matthieu Longo <matthieu.longo@arm.com> writes:
> 
>> On Linux, /proc/<pid> is keyed by the thread-group leader PID. When
>> the leader has exited, some /proc/<pid>/... entries become unavailable
>> even though another thread is still alive. This can happen, for instance,
>> when the main thread calls pthread_exit() and another thread continues
>> the execution (existing test: gcore-stale-thread).
>>
>> This causes GDB to fail to read procfs entries such as cmdline, cwd,
>> exe, maps, and smaps when it uses 'current_inferior ()->pid' after the
>> thread-group leader has exited.
>>
>> Fix this by adding inferior::first_non_exited_thread(), which returns
>> the PTID of the first non-exited thread of the inferior. Use its LWP ID
>> when accessing procfs entries that only need a representative live LWP
>> belonging to the process.
>>
>> This is a best-effort choice of a thread that is expected to still exist
>> in the target. Since GDB's view of the threads list may be stale, the
>> selected thread may already have exited by the time it is accessed.
>> Callers must therefore still be prepared to handle that case.
>>
>> Update the following functions:
>>  - linux_info_proc
>>  - linux_process_address_in_memtag_page
>>  - linux_find_memory_regions_full
>>  - linux_fill_prpsinfo
>>  - linux_address_in_shadow_stack_mem_range
>> to use the first non-exited thread's LWP ID instead of the inferior PID
>> when constructing procfs paths.
>>
>> Add a new test in gdb.threads.
>>
>> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31207
>> ---
>>  gdb/inferior.c                                | 12 +++
>>  gdb/inferior.h                                | 11 +++
>>  gdb/linux-tdep.c                              | 90 +++++++++++++------
>>  ...access-procfs-while-thread-leader-exited.c | 48 ++++++++++
>>  ...cess-procfs-while-thread-leader-exited.exp | 78 ++++++++++++++++
>>  5 files changed, 211 insertions(+), 28 deletions(-)
>>  create mode 100644 gdb/testsuite/gdb.threads/access-procfs-while-thread-leader-exited.c
>>  create mode 100644 gdb/testsuite/gdb.threads/access-procfs-while-thread-leader-exited.exp
> 
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> 
> Just one nit:
> 
>> diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
>> index 8c53ffd5e89..9bdcc55a0e1 100644
>> --- a/gdb/linux-tdep.c
>> +++ b/gdb/linux-tdep.c
>> @@ -457,6 +457,44 @@ linux_has_shared_address_space (struct gdbarch *gdbarch)
>>    return linux_is_uclinux ();
>>  }
>>  
>> +/* Return a PTID that identifies the current process and can be used to
>> +   access procfs safely.
>> +
>> +   The returned PTID is that of the thread-group leader whenever it is
>> +   still alive.  If the leader has already exited, the PTID of the first
>> +   non-exited thread in the current inferior is returned instead.
>> +   This ensures that the returned PTID always refers to a live thread
>> +   whose procfs entries are present and populated.  */
>> +static ptid_t
>> +get_process_reference_ptid (bool verbose = false)
>> +{
>> +  /* Get the current thread.  */
>> +  thread_info *thr = inferior_thread ();
>> +
>> +  /* Construct the PTID of the thread-group leader.  On Linux,
>> +     the leader's LWP ID is equal to the process ID.  */
>> +  ptid_t leader_ptid (thr->ptid.pid (), thr->ptid.pid ());
>> +
>> +  /* Use the thread-group leader if it is still alive.  Otherwise, use
>> +     the first thread that has not exited.  */
>> +  thread_info *leader_thr
>> +    = current_inferior ()->find_thread (leader_ptid);
>> +  ptid_t ptid = (leader_thr == nullptr
>> +		 ? current_inferior ()->first_non_exited_thread ()
>> +		 : leader_ptid);
>> +
>> +  if (!verbose)
>> +    return ptid;
>> +
>> +  if (leader_thr != nullptr)
>> +    gdb_printf (_("process %d\n"), leader_ptid.pid ());
>> +  else
>> +    gdb_printf (_("process %d [Note: information where gathered from LWP %ld " \
> 
> s/where/was/
> 
> Though actually IMHO it reads just as well with s/where //.
> 

Fixed.

Matthieu