[PATCH v2] elf: Fix GL(dl_phdr) and GL(dl_phnum) for static builds [BZ #29864]

[Adhemerval Zanella Netto]

On 02/02/23 06:36, John Paul Adrian Glaubitz wrote:
> Hi Adhemveral!
> 
> On Thu, 2023-01-12 at 11:44 -0300, Adhemerval Zanella wrote:
>> The 73fc4e28b9464f0e refactor did not add the GL(dl_phdr) and
>> GL(dl_phnum) for static build, relying on the __ehdr_start symbol,
>> which is always added by the static linker, to get the correct values.
>>
>> This is problematic in some ways:
>>
>>   - The segment may see its in-memory size differ from its in-file
>>     size (or the binary may have holes).  The Linux has fixed is to
>>     provide concise values for both AT_PHDR and AT_PHNUM (commit
>>     0da1d5002745c - "fs/binfmt_elf: Fix AT_PHDR for unusual ELF files")
>>
>>   - Some archs (alpha for instance) the hidden weak reference is not
>>     correctly pulled by the static linker and  __ehdr_start address
>>     end up being 0, which makes GL(dl_phdr) and GL(dl_phnum) have both
>>     invalid values (and triggering a segfault later on libc.so while
>>     accessing TLS variables).
>>
>> The safer fix is to just restore the previous behavior to setup
>> GL(dl_phdr) and GL(dl_phnum) for static based on kernel auxv.  The
>> __ehdr_start fallback can also be simplified by not assuming weak
>> linkage (as for PIE).
>>
>> The libc-static.c auxv init logic is moved to dl-support.c, since
>> the later is build without SHARED and then GLRO macro is defined
>> to access the variables directly.
>>
>> The _dl_phdr is also assumed to be always non NULL, since an invalid
>> NULL values does not trigger TLS initialization (which is used in
>> various libc systems).
>>
>> Checked on aarch64-linux-gnu, x86_64-linux-gnu, and i686-linux-gnu.
>>
>> * Changes from v1:
>>   - Removed auxv_values implicit initialization.
>>   - Removed unused header inclusion.
> 
> Any chance this fix can be backported to 2.36?

Done.