clayborg added a comment.
In D158583#4609320 <https://reviews.llvm.org/D158583#4609320>, @DavidSpickett
wrote:
> I'm not familiar with this mechanism but just out of curiosity: `ld.so loads
> the main executable and any dependent shared libraries and wants to update
> the "_r_debug" structure, but it now finds "_r_debug" in the a.out program
> and updates the state in this other copy`
>
> Is this some undefined behaviour or is there a good use case for this? From
> the program's point of view.
This is just how name lookups happen across shared library boundaries as far as
I know. External symbols get resolved by searching the shared libraries
including the main executable.
I have verified this is indeed what is happening by debugging the ld.so binary
and stepping through it at this code in rtld.c:
/* Notify the debugger all new objects are now ready to go. We must re-get
the address since by now the variable might be in another object. */
r = _dl_debug_initialize (0, LM_ID_BASE);
r->r_state = RT_CONSISTENT;
_dl_debug_state ();
The code for _dl_debug_initialize() looks like:
struct r_debug *
_dl_debug_initialize (ElfW(Addr) ldbase, Lmid_t ns)
{
struct r_debug *r;
if (ns == LM_ID_BASE)
r = &_r_debug; /// <- this will switch to
else
r = &GL(dl_ns)[ns]._ns_debug;
if (r->r_map == NULL || ldbase != 0)
{
/* Tell the debugger where to find the map of loaded objects. */
r->r_version = 1 /* R_DEBUG_VERSION XXX */;
r->r_ldbase = ldbase ?: _r_debug.r_ldbase;
r->r_map = (void *) GL(dl_ns)[ns]._ns_loaded;
r->r_brk = (ElfW(Addr)) &_dl_debug_state;
}
return r;
}
And the line "r = &_r_debug;" picks up the first item in the library search
paths for "_r_debug" which gets the one from the a.out program...
================
Comment at:
lldb/source/Plugins/DynamicLoader/POSIX-DYLD/DYLDRendezvous.cpp:260-266
+ // the dynamic loader. The problem happens as soon as the executable or
+ // shared library that exports another "_r_debug" is loaded by the dynamic
+ // loader due to the way symbols are found using the shared library search
+ // paths will mean that the new copy takes precedence over the one in the
+ // dynamic loader and the dynamic loader will be updating the other copy
+ // somewhere else that we won't find and that copy will have the
+ // eConsistent state.
----------------
DavidSpickett wrote:
> DavidSpickett wrote:
> > Can we split this sentence just for readability? Bullet points of each step
> > might be clearer.
> What you have in the commit message basically.
yeah, I was tired last night when finishing this up, I will grab the stuff from
the commit message
================
Comment at:
lldb/test/API/functionalities/dyld-multiple-rdebug/TestDyldWithMultupleRDebug.py:22
+ @no_debug_info_test
+ @skipIf(oslist=["linux"], archs=["arm"])
+ def test(self):
----------------
DavidSpickett wrote:
> Any specific reason for this? Not that it really matters, it'll get plenty of
> testing elsewhere.
Yeah, copy paste issue. I would be able to remove this.
================
Comment at:
lldb/test/API/functionalities/dyld-multiple-rdebug/TestDyldWithMultupleRDebug.py:28
+ exe = self.getBuildArtifact("a.out")
+ print(exe)
+ target = self.dbg.CreateTarget(exe)
----------------
DavidSpickett wrote:
> Leftover debug print.
good catch, yes!
================
Comment at:
lldb/test/API/functionalities/dyld-multiple-rdebug/TestDyldWithMultupleRDebug.py:54
+ self.assertIn("main",
+ thread.GetFrameAtIndex(0).GetDisplayFunctionName())
+ process.Continue()
----------------
aprantl wrote:
> You should be able to shorten the test setup considerably by using
> lldbutil.run_to_name_breakpoint() and only manually setting the second
> breakpoint afterwards.
will do
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D158583/new/
https://reviews.llvm.org/D158583
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