Re: Switching x86-64 to GNU2 TLS descriptors

[Adhemerval Zanella Netto via Gcc]

On 07/07/25 05:37, Florian Weimer via Gcc wrote:
> H.J. proposed to switch the default for GCC 16 (turning on
> -mtls-dialect=gnu2 by default).  This is a bit tricky because when we
> tried to make the switch in Fedora (for eventual implementation), we hit
> an ABI compatibility problem:
> 
>   _dl_tlsdesc_dynamic doesn't preserve all caller-saved registers
>   <https://sourceware.org/bugzilla/show_bug.cgi?id=31372>
> 
> This means that changing the defaults can have backwards compatibility
> impact with older distributions.
> 
> (a) Do not nothing special and switch the default.  Maybe try to
> backport the glibc fix to more release branches and distributions.  I
> think we implicitly decided to follow this path when we decided thiswas
> a glibc bug and not a GCC bug.  The downside is that missing the bug fix
> can result in unexpected, difficult-to-diagnose behavior.  However, when
> we rebuilt Fedora, the problem was exceedingly rare (we observed one
> single failure, if I recall correctly).
> 
> (b) Introduce binary markup to indicate that binaries may need the glibc
> fix, and that glibc has the fix.
> 
>   [PATCH] x86-64: Add GLIBC_ABI_GNU2_TLS [BZ #33129]
>   
> <https://inbox.sourceware.org/libc-alpha/20250704205341.155335-1-hjl.to...@gmail.com/>
> 
> This requires changes to all linkers, GCC and glibc.
> 
> (c) Introduce a new relocation type with the same behavior as
> R_X86_64_TLSDESC.  Unpatched glibc will not support it and error out
> during relocation processing.  Requires linker changes, GCC and glibc
> changes.  Does not produce a nice error message, unlike the
> GLIBC_ABI_GNU2_TLS change.  Ideally would need package manager changes
> to produce the right dependencies (with GLIBC_ABI_GNU2_TLS, this could
> happen automatically).
> 
> (d) Make the GCC default conditional on the glibc version used at GCC
> build time.  Add __memcmpeq support to GCC 16.  Maybe add
> errno@@GLIBC_2.43 to glibc 2.43.  Even today, it is likely that binaries
> contain at least one symbol version reference to something that is
> relatively recent, and the __memcmpeq and errno changes would increase
> this effect.  Combined with the backport mentioned under (a), that could
> be enough to force glibc upgrades in pretty much all cases.  We have
> __libc_start_main@@GLIBC_2.34, so if the glibc backports go back to 2.34
> (or even 2.31), only shared objects suffer from this issue.  Among the
> Fedora binaries, the outliers without dependencies on recent glibc are
> mostly Perl modules, and I expect the errno and __memcmpeq would cover
> at least some of these.  This is not as clean as (b) and (c), but only
> needs glibc and GCC changes (for __memcmpeq).  It does not achieve 100%
> bug prevention, but given that bugs seem to be rare, this may be good
> enough.
> 
> (e) Skip over GNU2 TLS altogether and implement inline TLS sequences
> (GNU3 descriptors?) that do not have the dlopen incompatibility of
> initial-exec TLS.  This is currently vaporware.  It requires nontrivial
> glibc changes, GCC changes, linker changes, and x86-64 psABI work to
> define new relocation types and perhaps relaxations.  This is probably
> what we want long-term.  User experience is similar to (c), but with
> more implementation sequences.
> 
> For comparison with an initial-exec TLS read,
> 
>       movq    threadvar@gottpoff(%rip), %rax
>       movl    %fs:(%rax), %eax
> 
> this could look like this:
> 
>       movl    threadvar@gottpslot, %eax
>         movq  %fs:(%rax), %rax
>         movl  threadvar@gottlsslotoff, %ecx
>         movl  (%rcx, %rax), %eax
> 
> Or with the descriptor in one word:
> 
>       movq    threadvar@gottpslotoff, %rax
>         movq  %rax, %rdx
>         movq  %fs:(%eax), %rax
>         shrq  $32, %rdx
>         movl  %(rax, %rdx), %eax
> 
> Or with a bit shorter instruction, using a 32-bit descriptor (which
> still could cover at least 3 GiB of TLS data per thread):
> 
>       movl    threadvar@gottpslotoff, %rax
>         movzbl        %al, %edx
>         shr   $8, %eax
>         movq  %fs:64(%edx), %rdx
>         mov   (%rdx, %rax), %eax
> 
> And if we want a negative TLS slot index (which glibc would not use, and
> I think it's incompatible with local-exec TLS anyway):
> 
>       movq    threadvar@gottpslotoff, %rax
>         movslq        %eax, %rdx
>         shrq  $32, %rax
>         movq  %fs:(%rdx), %rdx
>         movl  %(rdx, %rax), %eax
> 
> There might be other variant sequences.
> 
> Implementing this on the glibc side would require fundamental changes to
> the TLS allocator, which is why this isn't straightforward.
> 
> (f) A less ambitions variant of (e): A new TLS descriptor call back that
> returns the address of the TLS variable, and not the offset from the
> thread pointer.  This is much easier to implement on the glibc side.
> The current GNU2 TLS descriptor callback is optimized for static TLS
> access.  We can avoid a memory access in the static TLS callback if we
> use the RDFSBASE instruction (if glibc detects run-time support).  It's
> a new relocation type, so this too needs GCC, linker, ABI changes.
> However, these changes are largely mechanical (except perhaps for the
> relaxation support).  Basically, TLS accesses would change from
> 
>       leaq    threadvar@TLSDESC(%rip), %rax
>       call    *threadvar@TLSCALL(%rax)
>       movl    %fs:(%rax), %eax
> 
> to:
> 
>       leaq    threadvar@TLSDESC2(%rip), %rax
>       call    *threadvar@TLSCALL2(%rax)
>       movl    (%rax), %eax
> 
> And the implementation of the static TLS case would change from
> 
>       endbr64
>         movq  8(%rax), %rax
>         retq
> 
> to:
> 
>       endbr64
>         rdfsbase %rax
>         addq  %rsi, %rax
>         retq
> 
> But I don't think this detour is worth it if we eventually want to land
> on (e).
> 
> 
> I'm personally leaning towards (d) or (a) for GCC 16.  I dislike (b).
> And (e) is unrealistic in the short term.

We did something similar to (b) for DT_RELR, so it is not unprecedented.

The (b) might generate some attrition when users try to deploy binaries
built with recent gcc on older systems; even though running with the new
TLS variant is subject to breakage.  Either users will fallback to use
the old tls dialect or employ some hacks like remove the 
GLIBC_ABI_GNU2_TLS mark (and yeah, there are multiple cases like this,
like Firefox with DT_RELR).  But at least the toolchain does provide
a way to work around it.

As H.J has noted, GLIBC_ABI_GNU2_TLS only requires glibc and linker
issues.  This does not help when recent gcc are used with old binutils,
but I take this is not really a common setup.

The (d) can also work, but different than __libc_start_main any other
symbols might not create a direct dependency and thus this is a partial
fix.

I am leaning more to (b).