[PATCH] gdb: relax requirement for the map_failed stap probe to be present

[Andrew Burgess <aburgess@redhat.com>]

From glibc 2.35 and later, the "map_failed" stap probe is no longer
included in glibc.  The removal of the probe looks like an accident,
but it was caused by a glibc commit which meant that the "map_failed"
probe could no longer be reached; the compiler than helpfully
optimised out the probe.

In GDB, in solib-svr4.c, we have a list of probes that we look for
related to the shared library loading detection.  If any of these
probes are missing then GDB will fall back to the non-probe based
mechanism for detecting shared library loading.  The "map_failed"
probe is include in the list of required probes.

This means that on glibc 2.35 (or later) systems, GDB is going to
always fall back to the non-probes based mechanism for detecting
shared library loading.

I raised a glibc bug to discuss this issue:

  https://sourceware.org/bugzilla/show_bug.cgi?id=29818

But, whatever the ultimate decision from the glibc team, given there
are version of glibc in the wild without the "map_failed" probe, we
probably should update GDB to handle this situation.

The "map_failed" probe is already a little strange, very early
versions of glibc didn't include this probe, so, in some cases, if
this probe is missing GDB is happy to ignore it.  In this commit I
just expand this logic to make the "map_failed" probe fully optional.

With this commit in place, then, when using a glibc 2.35 or later
system, GDB will once again use the stap probes for shared library
detection.
---
 gdb/solib-svr4.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index 6acaf87960b..87cd06f251a 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -2205,10 +2205,15 @@ svr4_find_and_create_probe_breakpoints (svr4_info *info,
 
       probes[i] = find_probes_in_objfile (os->objfile, "rtld", name);
 
-      /* The "map_failed" probe did not exist in early
-	 versions of the probes code in which the probes'
-	 names were prefixed with "rtld_".  */
-      if (with_prefix && streq (name, "rtld_map_failed"))
+      /* The "map_failed" probe did not exist in early versions of the
+	 probes code in which the probes' names were prefixed with
+	 "rtld_".
+
+	 Additionally, the "map_failed" probe was accidentally removed
+	 from glibc 2.35 and later, when changes in glibc meant the probe
+	 could no longer be reached.  In this case the probe name doesn't
+	 have the "rtld_" prefix.  */
+      if (streq (probe_info[i].name, "map_failed"))
 	continue;
 
       /* Ensure at least one probe for the current name was found.  */

base-commit: 84f9fbe90e5429adb9dee68f04f44c92fa9e2345
-- 
2.25.4