On Mon, Jul 7, 2025 at 4:37 PM Florian Weimer <fwei...@redhat.com> 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.
This option is independent of GCC. Only glibc and linker changes are needed. It just introduces a glibc version dependency whenever GNU2 TLS is used, regardless whether it is the default or not. > (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. > > Thanks, > Florian > -- H.J.
