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

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

On Tue, Apr 9, 2024 at 7:11 AM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Tue, 09 Apr 2024 07:03:45 PDT (-0700), 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.
> >
> > It also means to document it properly somewhere and make the new
> > RISC-V and loongarch follow the same guidelines.
>
> I also don't have a strong opinion on whether __tls_get_addr should be
> interposable, but I'm happy to try and make the RISC-V port match
> arm64/x86.  I guess we're kind of safe for now as we don't have TLSDESC
> merged, though I think we were getting pretty close there so we should
> probbaly decide before we accidentally commit to an ABI.
>
> > I will take a look again on the ThreadPropertiesAPI, since it is has
> > been more and more a demanding issue.

We should add a __tls_get_addr glibc test if we decide it should be
called via PLT.

-- 
H.J.