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

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

[Andrew Burgess]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=79d403654266c747703359e4b4e7f3931fb53f99

commit 79d403654266c747703359e4b4e7f3931fb53f99
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Tue Nov 22 12:45:56 2022 +0000

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

Diff:
---
 gdb/solib-svr4.c | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index 6acaf87960b..0fe6c239c66 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -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])