[PATCH v3] Detect ld.so and libc.so version inconsistency during startup

[Florian Weimer]

* Carlos O'Donell:

> On 8/23/22 17:31, Florian Weimer wrote:
>> The files NEWS, include/link.h, and sysdeps/generic/ldsodefs.h
>> contribute to the version fingerprint used for detection.  The
>> fingerprint can be further refined using the --with-extra-version-id
>> configure argument.
>> 
>> _dl_call_libc_early_init is replaced with _dl_lookup_libc_early_init.
>> The new function is used store a pointer to libc.so's
>> __libc_early_init function in the libc_map_early_init member of the
>> ld.so namespace structure.  This function pointer can then be called
>> directly, so the separate invocation function is no longer needed.
>> 
>> The versioned symbol lookup needs the symbol versioning data
>> structures, so the initialization of libc_map and libc_map_early_init
>> is now done from _dl_check_map_versions, after this information
>> becomes available.  (_dl_map_object_from_fd does not set this up
>> in time, so the initialization code had to be moved from there.)
>> This means that the separate initialization code can be removed from
>> dl_main because _dl_check_map_versions covers all maps, including
>> the initial executable loaded by the kernel.  The lookup still happens
>> before relocation and the invocation of IFUNC resolvers, so IFUNC
>> resolvers are protected from ABI mismatch.
>> 
>> The __libc_early_init function pointer is not protected because
>> so little code runs between the pointer write and the invocation
>> (only dynamic linker code and IFUNC resolvers).
>
> This is really an exciting change since it adds a diagnostic and clean exit
> for a case that was previously a mysterious crash. Thanks for working through
> this change.
>
> v3 LGTM.
>
> Tested-by: Carlos O'Donell <carlos@redhat.com>
> Reviewed-by: Carlos O'Donell <carlos@redhat.com>

Thanks.  I can't merge this as-is because we have

| if not hasattr(enum, 'IntFlag'):
|     import sys
|     sys.stdout.write(
|         'warning: glibcelf.py needs Python 3.6 for enum support\n')
|     sys.exit(77)

in scripts/glibcelf.py.  We use glibcelf.py at build time with this
patch, so this will move the minumum Python requirement to 3.6.  I guess
I should have added the hash function directly to the new script.

I'll see if I can clean up scripts/glibcelf.py today and make it
compatible with earlier Python versions.

Thanks,
Florian