Avoid /proc/pid/mem races (PR 28065)

Avoid /proc/pid/mem races (PR 28065)

PR 28065 (gdb.threads/access-mem-running-thread-exit.exp intermittent
failure) shows that GDB can hit an unexpected scenario -- it can
happen that the kernel manages to open a /proc/PID/task/LWP/mem file,
but then reading from the file returns 0/EOF, even though the process
hasn't exited or execed.

"0" out of read/write is normally what you get when the address space
of the process the file was open for is gone, because the process
execed or exited.  So when GDB gets the 0, it returns memory access
failure.  In the bad case in question, the process hasn't execed or
exited, so GDB fails a memory access when the access should have
worked.

GDB has code in place to gracefully handle the case of opening the
/proc/PID/task/LWP/mem just while the LWP is exiting -- most often the
open fails with EACCES or ENOENT.  When it happens, GDB just tries
opening the file for a different thread of the process.  The testcase
is written such that it stresses GDB's logic of closing/reopening the
/proc/PID/task/LWP/mem file, by constantly spawning short lived
threads.

However, there's a window where the kernel manages to find the thread,
but the thread exits just after and clears its address space pointer.
In this case, the kernel creates a file successfully, but the file
ends up with no address space associated, so a subsequent read/write
returns 0/EOF too, just like if the whole process had execed or
exited.  This is the case in question that GDB does not handle.

Oleg Nesterov gave this suggestion as workaround for that race:

    gdb can open(/proc/pid/mem) and then read (say) /proc/pid/statm.
    If statm reports something non-zero, then open() was "successfull".

I think that might work.  However, I didn't try it, because I realized
we have another nasty race that that wouldn't fix.

The other race I realized is that because we close/reopen the
/proc/PID/task/LWP/mem file when GDB switches to a different inferior,
then it can happen that GDB reopens /proc/PID/task/LWP/mem just after
a thread execs, and before GDB has seen the corresponding exec event.
I.e., we can open a /proc/PID/task/LWP/mem file accessing the
post-exec address space thinking we're accessing the pre-exec address
space.

A few months back, Simon, Oleg and I discussed a similar race:

  [Bug gdb/26754] Race condition when resuming threads and one does an exec
  https://sourceware.org/bugzilla/show_bug.cgi?id=26754

The solution back then was to make the kernel fail any ptrace
operation until the exec event is consumed, with this kernel commit:

 commit dbb5afad100a828c97e012c6106566d99f041db6
 Author:     Oleg Nesterov <oleg@redhat.com>
 AuthorDate: Wed May 12 15:33:08 2021 +0200
 Commit:     Linus Torvalds <torvalds@linux-foundation.org>
 CommitDate: Wed May 12 10:45:22 2021 -0700

     ptrace: make ptrace() fail if the tracee changed its pid unexpectedly

This however, only applies to ptrace, not to the /proc/pid/mem file
opening case.  Also, even if it did apply to the file open case, we
would want to support current kernels until such a fix is more wide
spread anyhow.

So all in all, this commit gives up on the idea of only ever keeping
one /proc/pid/mem file descriptor open.  Instead, make GDB open a
/proc/pid/mem per inferior, and keep it open until the inferior exits,
is detached or execs.  Make GDB open the file right after the inferior
is created or is attached to or forks, at which point we know the
inferior is stable and stopped and isn't thus going to exec, or have a
thread exit, and so the file open won't fail (unless the whole process
is SIGKILLed from outside GDB, at which point it doesn't matter
whether we open the file).

This way, we avoid both races described above, at the expense of using
more file descriptors (one per inferior).

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28065
Change-Id: Iff943b95126d0f98a7973a07e989e4f020c29419