[PATCH] nptl: Export libthread_db-used symbols under GLIBC_PRIVATE
Simon Marchi
simon.marchi@polymtl.ca
Tue Jun 29 14:20:32 GMT 2021
On 2021-06-27 6:25 p.m., Carlos O'Donell wrote:> On 6/20/21 8:38 AM, Simon Marchi via Libc-alpha wrote:
>> On 2021-06-17 3:23 p.m., Florian Weimer via Gdb wrote:
>>> Kevin, this *almost* gives us the perfect debugging experience with your
>>> patched GDB in Fedora 35, according to my preliminary tests. A build
>>> with the patch is running here:
>>>
>>> <https://koji.fedoraproject.org/koji/buildinfo?buildID=1773177>
>>>
>>> However, it seems that GDB still needs pthread_create in the .symtab to
>>> trigger loading of libpthread_db. A fully stripped libc.so.6 without
>>> .symtab does not trigger loading in my experiments. (The other symbols
>>> we preserve for valgrind's sake and are immaterial to GDB.) In other
>>> words,
>>
>> Stupid question, but: since pthread_create is a function exposed by the
>> libc.so shared library, won't there still be an entry for it in .dynsym?
>
> No question is stupid. Thank you for asking.
>
> Yes, there *must* be a .dynsym entry for pthread_create in libc.so.6 in
> order for the dynamic loader to bind the reference to the definition.
>
>> And if so, shouldn't GDB see it?
>
> It should. Today gdb is only using the internal lookup_minimal_symbol()
> API in order to trigger libthread_db loading, and it's not clear to me
> if that will load *all* of the dynamic symbols during gdb startup.
>
> Someone would need to debug the minsyms.c API to see if something isn't
> working as expected.
>
> gdb/minsyms.c:
>
> 1267 When files contain multiple sources of symbol information, it is
> 1268 possible for the minimal symbol table to contain many duplicate entries.
> 1269 As an example, SVR4 systems use ELF formatted object files, which
> 1270 usually contain at least two different types of symbol tables (a
> 1271 standard ELF one and a smaller dynamic linking table), as well as
> 1272 DWARF debugging information for files compiled with -g.
>
> So it sounds like the data is loadable, and mergeable, but it appears that
> it's not all there at early startup in gdb.
My intuition is that the .dynsym symbols are normally loaded and result
in minimal symbols in GDB:
https://gitlab.com/gnutools/gdb/-/blob/master/gdb/elfread.c#L1102-1128
But there may be some filtering that prevents the symbols like
pthread_create from making their way into the minimal symbols database.
I'll have to try using the container that Florian pointed to
(registry.fedoraproject.org/fedora:rawhide).
Simon
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