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

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

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).