Re: [PATCH] aarch64: Remove ld.so __tls_get_addr plt usage

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Apr 9, 2024 at 7:03 AM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 09/04/24 05:30, Szabolcs Nagy wrote:
> > The 04/08/2024 13:57, Adhemerval Zanella Netto wrote:
> >> On 08/04/24 04:26, Szabolcs Nagy wrote:
> >>> The 04/07/2024 16:29, Cristian Rodríguez wrote:
> >>>> On Fri, Apr 5, 2024 at 11:59 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
> >>>>> The 04/05/2024 09:35, Adhemerval Zanella wrote:
> >>>>>> Use the hidden alias instead.
> >>>>>>
> >>>>>> Checked on aarch64-linux-gnu.
> >>>>>
> >>>>> does this change behaviour in case __tls_get_addr is interposed?
> >>>>
> >>>> Wut ? is that really supported.. I mean.. isn't that symbol prefix
> >>>> reserved for the implementation and any assumption about it is either
> >>>> ID or UB?
> >>>
> >>> a behaviour can change even if it's not supported.
> >>> i did not try to imply that it should be supported.
> >>>
> >>> i know sanitizers interpose __tls_get_addr, because
> >>> https://sourceware.org/bugzilla/show_bug.cgi?id=16291
> >>> i don't know if that hack works at all now for tlsdesc
> >>> (where the ld.so calls __tls_get_addr, not user code)
> >>>
> >>> my question was if we investigated this issue since it
> >>> is useful to document then in the commit msg (or news
> >>> entry if this affects users)
> >>
> >> This change 'breaks' the sanitizer trick to get the dynamic TLS, with
> >> this patch I now see:
> >>
> >>   MemorySanitizer-AARCH64 :: dtls_test.c
> >>   SanitizerCommon-asan-aarch64-Linux :: Linux/resize_tls_dynamic.cpp
> >>   SanitizerCommon-msan-aarch64-Linux :: Linux/resize_tls_dynamic.cpp
> >>   SanitizerCommon-tsan-aarch64-Linux :: Linux/resize_tls_dynamic.cpp
> >>
> >> And it does not fail on x86 only because it uses -mtls=gnu as default
> >> (the same tests fail on x86 with -mtls=gnu2).
> >>
> >> Now that GCC and distributions are aiming to use GNU2/DESC as the
> >> default TLS, this hack will also break on x86.  So the question is
> >> whether we revert 050f7298e1ecc39887c329037575ccd972071255 and
> >> document that __tls_get_addr should be interposable, or move with this
> >> change and try to come up with a possible solution for BZ#16291.
> >>
> >> I bringing this because we will have another two ABIs with tlsdesc
> >> support (loongarch and riscv).
> >
> > adding some sanitizer committers to cc.
> >
> > tl;dr: in the next glibc release tlsdesc will not call
> > __tls_get_addr in an interposable way in the dynamic tls
> > allocation case, unless somebody screems that this is needed.
> > (affects targets that may default to tlsdesc, but note that
> > the dynamic case only triggers with tlsdesc when a lot of
> > dlopened tls is used, otherwise static tls area is used)
>
> Just a note that this already true for x86 with -mtls=gnu2 since
> 2.21.  And now that distro are aiming to make it default, this issues
> will happen more often.
>
> >
> > i think it is also possible that we will use custom malloc
> > in ld.so which may be just as big change for the sanitizers.
> > (this can make tls access signal safe)
> >
> > i'm not against the change, but if we plan to add several
> > interposable hooks as in
> > https://sourceware.org/glibc/wiki/ThreadPropertiesAPI
> > then we might as well keep __tls_get_addr PLT for now.
> >
>
> I don't have a strong opinion, but what I really want is to have
> consistency over the architectures.  Meaning that if we want to keep
> the __tls_get_addr PLT for sanitizer/runtime hooks, it would be good
> to revert the x86 change.

We need to add gnu2 tests to libsanitizer when making the x86 change.

> It also means to document it properly somewhere and make the new
> RISC-V and loongarch follow the same guidelines.
>
> I will take a look again on the ThreadPropertiesAPI, since it is has
> been more and more a demanding issue.



-- 
H.J.