[Bug cli/31352] New: [gdb/cli, recursive internal problem] sig_write uses gdb_stderr->write_async_safe, which may be a string_file

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

https://sourceware.org/bugzilla/show_bug.cgi?id=31352

            Bug ID: 31352
           Summary: [gdb/cli, recursive internal problem] sig_write uses
                    gdb_stderr->write_async_safe, which may be a
                    string_file
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: cli
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

I tried debugging a dap test-case (gdb.dap/pause.exp), by replacing:
...
@@ -661,13 +661,16 @@ quit (void)
 #else
   if (job_control
       /* If there is no terminal switching for this target, then we can't
         possibly get screwed by the lack of job control.  */
       || !target_supports_terminal_ours ())
-    throw_quit ("Quit");
+    __builtin_abort ();
   else
     throw_quit ("Quit (expect signal SIGINT when the program is resumed)");
 #endif
...
to try to produce a corefile.

I noticed this didn't produce a core file, but it did mention recursive
internal problems, so I decided to try a bit harder:
...
@@ -347,7 +347,7 @@ internal_vproblem (struct internal_problem *problem,
   /* Don't allow infinite error/warning recursion.  */
   {
     static const char msg[] = "Recursive internal problem.\n";
-
+    __builtin_abort ();
     switch (dejavu)
       {
       case 0:
...
and managed to produce a core file, due to a segfault.

The segfault is due to running out of stack, and the stack loop looks like:
...
gdb) 
#16321 0x00000000014f89f5 in internal_error_loc (file=0x160fac0
"/data/vries/gdb/src/gdb/ui-file.h", line=72, 
    fmt=0x160faa4 "%s: write_async_safe") at
/data/vries/gdb/src/gdbsupport/errors.cc:58
58        internal_verror (file, line, fmt, ap);
(gdb) down
#16320 0x0000000000d2433d in internal_verror (file=0x160fac0
"/data/vries/gdb/src/gdb/ui-file.h", line=72, 
    fmt=0x160faa4 "%s: write_async_safe", ap=0x7ffc703be958) at
/data/vries/gdb/src/gdb/utils.c:495
495       internal_vproblem (&internal_error_problem, file, line, fmt, ap);
(gdb) 
#16319 0x0000000000d24307 in internal_vproblem(internal_problem *, const char
*, int, const char *, typedef __va_list_tag __va_list_tag *) (problem=0x276b5e0
<internal_error_problem>, 
    file=0x160fac0 "/data/vries/gdb/src/gdb/ui-file.h", line=72, fmt=0x160faa4
"%s: write_async_safe", 
    ap=0x7ffc703be958) at /data/vries/gdb/src/gdb/utils.c:350
350         __builtin_abort ();
(gdb) 
#16318 0x00007f1a9e6553e5 in abort () from /lib64/libc.so.6
(gdb) 
#16317 0x00007f1a9e653d2b in raise () from /lib64/libc.so.6
(gdb) 
#16316 <signal handler called>
(gdb) 
#16315 0x00000000007a35eb in handle_fatal_signal (sig=6) at
/data/vries/gdb/src/gdb/event-top.c:898
898           sig_write ("\n\n");
(gdb) 
#16314 0x00000000007a35b1 in <lambda(char const*)>::operator()(const char *)
const (__closure=0x7ffc703bd8af, 
    msg=0x15ed81c "\n\n") at /data/vries/gdb/src/gdb/event-top.c:893
893         gdb_stderr->write_async_safe (msg, strlen (msg));
(gdb) 
#16313 0x000000000082e644 in ui_file::write_async_safe (this=0x7ffc703c1970,
buf=0x15ed81c "\n\n", length_buf=2)
    at /data/vries/gdb/src/gdb/ui-file.h:72
72        { gdb_assert_not_reached ("write_async_safe"); }
(gdb) 
#16312 0x00000000014f89f5 in internal_error_loc (file=0x160fac0
"/data/vries/gdb/src/gdb/ui-file.h", line=72, 
    fmt=0x160faa4 "%s: write_async_safe") at
/data/vries/gdb/src/gdbsupport/errors.cc:58
58        internal_verror (file, line, fmt, ap);
(gdb) 
...

AFAICT, what happens is:
- abort is raised
- abort is caught
- attempt to write backtrace using sig_write
- sigwrite does gdb_stderr->write_async_safe
- since gdb_stderr is set to a string_file, which doesn't have
  write_async_safe an internal_error is thrown
- the internal_error ends up calling the abort I added in internal_vproblem,
  and another abort is raised

This can easily be avoided by printing to stderr instead:
...
diff --git a/gdb/bt-utils.c b/gdb/bt-utils.c
index 6f68e269c51..f93e45688e8 100644
--- a/gdb/bt-utils.c
+++ b/gdb/bt-utils.c
@@ -56,7 +56,7 @@ libbacktrace_error (void *data, const char *errmsg, int
errnum)

   const auto sig_write = [] (const char *msg) -> void
   {
-    gdb_stderr->write_async_safe (msg, strlen (msg));
+    fprintf (stderr, "%s", msg);
   };

   sig_write ("error creating backtrace: ");
@@ -80,7 +80,7 @@ libbacktrace_print (void *data, uintptr_t pc, const char
*filename,
 {
   const auto sig_write = [] (const char *msg) -> void
   {
-    gdb_stderr->write_async_safe (msg, strlen (msg));
+    fprintf (stderr,"%s",  msg);
   };

   /* Buffer to print addresses and line numbers into.  An 8-byte address
@@ -131,7 +131,7 @@ gdb_internal_backtrace_1 ()
 {
   const auto sig_write = [] (const char *msg) -> void
   {
-    gdb_stderr->write_async_safe (msg, strlen (msg));
+    fprintf (stderr, msg);
   };e--

   /* Allow up to 25 frames of backtrace.  */
@@ -159,7 +159,7 @@ gdb_internal_backtrace ()
 #ifdef GDB_PRINT_INTERNAL_BACKTRACE
   const auto sig_write = [] (const char *msg) -> void
   {
-    gdb_stderr->write_async_safe (msg, strlen (msg));
+    fprintf (stderr, "%s", msg);
   };

   sig_write (_("----- Backtrace -----\n"));
diff --git a/gdb/event-top.c b/gdb/event-top.c
index 33aef7d7cc5..b3d16ecd710 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -890,7 +890,7 @@ handle_fatal_signal (int sig)
 #ifdef GDB_PRINT_INTERNAL_BACKTRACE
   const auto sig_write = [] (const char *msg) -> void
   {
-    gdb_stderr->write_async_safe (msg, strlen (msg));
+    fprintf (stderr, "%s", msg);
   };

   if (bt_on_fatal_signal)
...

With this patch, I can get rid of the abort in internal_vproblem and still get
my core dump.

I don't know what is a proper fix for this.

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