[Bug gdb/31528] New: [gdb, kernel.yama.ptrace_scope=1] FAIL: gdb.base/attach-deleted-exec.exp: attach to process with deleted executable

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31528

            Bug ID: 31528
           Summary: [gdb, kernel.yama.ptrace_scope=1] FAIL:
                    gdb.base/attach-deleted-exec.exp: attach to process
                    with deleted executable
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

When kernel.yama.ptrace_scope is set to 1, we have limited ptrace attach
capability, and run into:
...
(gdb) attach 27383^M
Attaching to process 27383^M
ptrace: Operation not permitted.^M
(gdb) FAIL: gdb.base/attach-deleted-exec.exp: attach to process with deleted
executable
...

We need to be either root or use some special commands to acquire ptrace
capability (see PR31520).

With root, we have:
...
(gdb) attach 28678^M
Attaching to process 28678^M
Reading symbols from /proc/28678/exe...^M
Reading symbols from /lib64/libm.so.6...^M
(No debugging symbols found in /lib64/libm.so.6)^M
Reading symbols from /lib64/libc.so.6...^M
(No debugging symbols found in /lib64/libc.so.6)^M
Reading symbols from /lib64/ld-linux-x86-64.so.2...^M
(No debugging symbols found in /lib64/ld-linux-x86-64.so.2)^M
0x00007fc13de420ca in clock_nanosleep@GLIBC_2.2.5 () from /lib64/libc.so.6^M
(gdb) PASS: gdb.base/attach-deleted-exec.exp: attach to process with deleted
executable
...

But with the script from PR31520, we run into:
...
(gdb) attach 29113^M
Attaching to process 29113^M
No executable file now.^M
warning: Could not load vsyscall page because no executable was specified^M
0x00007f92f00ae0ca in ?? ()^M
(gdb) FAIL: gdb.base/attach-deleted-exec.exp: attach to process with deleted
executable
...
because:
...
access("/proc/29113/exe", R_OK)         = -1 EACCES (Permission denied)
...

If we modify the code to skip the access check:
...
diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
index b17e3120792..2b890ce1387 100644
--- a/gdb/nat/linux-procfs.c
+++ b/gdb/nat/linux-procfs.c
@@ -352,9 +352,8 @@ linux_proc_pid_to_exec_file (int pid)
   else
     buf[len] = '\0';

-  /* Use /proc/PID/exe if the actual file can't be read, but /proc/PID/exe
-     can be.  */
-  if (access (buf, R_OK) != 0 && access (name, R_OK) == 0)
+  /* Use /proc/PID/exe if the actual file can't be read.  */
+  if (access (buf, R_OK) != 0)
     strcpy (buf, name);

   return buf;
...
the test passes:
...
(gdb) attach 29927^M
Attaching to process 29927^M
Reading symbols from /proc/29927/exe...^M
Reading symbols from /lib64/libm.so.6...^M
(No debugging symbols found in /lib64/libm.so.6)^M
Reading symbols from /lib64/libc.so.6...^M
(No debugging symbols found in /lib64/libc.so.6)^M
Reading symbols from /lib64/ld-linux-x86-64.so.2...^M
(No debugging symbols found in /lib64/ld-linux-x86-64.so.2)^M
0x00007f686bb670ca in clock_nanosleep@GLIBC_2.2.5 () from /lib64/libc.so.6^M
(gdb) PASS: gdb.base/attach-deleted-exec.exp: attach to process with deleted
executable
...

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