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

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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 then 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.  This is fine, the
action associated with this probe inside GDB is DO_NOTHING, this means
the probe isn't actually required in order for GDB to correctly detect
the loading of shared libraries.

In this commit I propose changing the rules so that any probe whose
action is DO_NOTHING, is optional.

There is one possible downside to this change, and that concerns 'set
stop-on-solib-events on'.  If a probe is removed from glibc, but the
old style breakpoint based mechanism is still in place within glibc
for that same event, then GDB will stop when using the old style
non-probe based mechanism, but not when using the probes based
mechanism.

For the map_failed case this is not a problem, both the map_failed
probe, and the call to the old style breakpoint location were
optimised out, and so neither event (probes based, or breakpoint
based) will trigger.  This would only become an issue if glibc removed
a probe, but left the breakpoint in place (this would almost certainly
be a bug in glibc).

For now, I'm proposing that we just don't worry about this.  Because
some probes have actions that are not DO_NOTHING, then we know the
user will always seem _some_ stops when a shared library is
loaded/unloaded, and (I'm guessing), in most cases, that's all they
care about.  I figure when someone complains then we can figure out
what the right solution is then.

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.

Reviewed-By: Lancelot SIX <lancelot.six@amd.com>
This commit is contained in:
Andrew Burgess 2022-11-22 12:45:56 +00:00
parent 149700668d
commit 79d4036542
1 changed files with 26 additions and 7 deletions
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33
gdb/solib-svr4.c
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@ -2205,15 +2205,34 @@ 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"))
continue;
/* Ensure at least one probe for the current name was found. */
if (probes[i].empty ())
return false;
{
/* 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 2.36, when changes in glibc meant the
probe could no longer be reached, and the compiler optimized
the probe away. In this case the probe name doesn't have the
"rtld_" prefix.
To handle this, and give GDB as much flexibility as possible,
we make the rule that, if a probe isn't required for the
correct operation of GDB (i.e. its action is DO_NOTHING), then
we will still use the probes interface, even if that probe is
missing.
The only (possible) downside of this is that, if the user has
'set stop-on-solib-events on' in effect, then they might get
fewer events using the probes interface than with the classic
non-probes interface. */
if (probe_info[i].action == DO_NOTHING)
continue;
else
return false;
}
/* Ensure probe arguments can be evaluated. */
for (probe *p : probes[i])