[PATCH][gdb/tdep] Use pid to choose x86_64 process 64/32-bitness

[Tom de Vries <tdevries@suse.de>]

Hi,

In a linux kernel mailing list discussion, it was mentioned that "gdb has
this odd thing where it takes the 64-bit vs 32-bit data for the whole process
from one thread, and picks the worst possible thread to do it (ie explicitly
not even the main thread, ...)" [1].

The picking of the thread is done here in
x86_linux_nat_target::read_description:
...
  /* GNU/Linux LWP ID's are process ID's.  */
  tid = inferior_ptid.lwp ();
  if (tid == 0)
    tid = inferior_ptid.pid (); /* Not a threaded program.  */
...

To understand what this code does, let's investigate a scenario in which
inferior_ptid.lwp () != inferior_ptid.pid ().

Say we start exec jit-attach-pie, identified with pid x.  The main thread
starts another thread that sleeps, and then the main thread waits for the
sleeping thread.  So we have two threads, identified with LWP IDs x and x+1:
...
PID  LWP  CMD
x    x    ./jit-attach-pie
x    x+1  ./jit-attach-pie
...
[ The thread with LWP x is known as the thread group leader. ]

When attaching to this exec using the pid, gdb does a stop_all_threads which
iterates over all the threads, first LWP x, and then LWP x+1.

So the state we arrive with at x86_linux_nat_target::read_description is:
...
(gdb) p inferior_ptid
$1 = {m_pid = x, m_lwp = x+1, m_tid = 0}
...
and consequently we probe 64/32-bitness from thread LWP x+1.

[ Note that this is different from when gdb doesn't attach but instead
launches the exec itself, in which case there's no stop_all_threads needed,
and the probed thread is LWP x. ]

According to aforementioned remark, a better choice would have been the main
thread, that is, LWP x.

This patch implement that choice, by simply doing:
...
  tid = inferior_ptid.pid ();
...

The fact that gdb makes a per-process choice for 64/32-bitness is a problem in
itself: each thread can be in either 64 or 32 bit mode.  That is a problem
that this patch doesn't fix.

Tested on x86_64-linux.

[1] https://lore.kernel.org/io-uring/\
  CAHk-=wh0KoEZXPYMGkfkeVEerSCEF1AiCZSvz9TRrx=Kj74D+Q@mail.gmail.com/

Any comments?

Thanks,
- Tom

[gdb/tdep] Use pid to choose x86_64 process 64/32-bitness

gdb/ChangeLog:

2021-05-07  Tom de Vries  <tdevries@suse.de>

	PR tdep/27822
	* x86-linux-nat.c (x86_linux_nat_target::read_description): Use
	pid to determine if process is 64-bit or 32-bit.

---
 gdb/x86-linux-nat.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/gdb/x86-linux-nat.c b/gdb/x86-linux-nat.c
index 85c7f0ddc94..adea1ad0092 100644
--- a/gdb/x86-linux-nat.c
+++ b/gdb/x86-linux-nat.c
@@ -113,10 +113,7 @@ x86_linux_nat_target::read_description ()
   static uint64_t xcr0;
   uint64_t xcr0_features_bits;
 
-  /* GNU/Linux LWP ID's are process ID's.  */
-  tid = inferior_ptid.lwp ();
-  if (tid == 0)
-    tid = inferior_ptid.pid (); /* Not a threaded program.  */
+  tid = inferior_ptid.pid ();
 
 #ifdef __x86_64__
   {