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

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella:

> 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.

This version looks okay to me.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

Thanks,
Florian