From: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
To: Florian Weimer <fweimer@redhat.com>, Fangrui Song <maskray@google.com>
Cc: "Szabolcs Nagy" <szabolcs.nagy@arm.com>,
"Cristian Rodríguez" <cristian@rodriguez.im>,
"H.J. Lu" <hjl.tools@gmail.com>,
libc-alpha@sourceware.org, "Vitaly Buka" <vitalybuka@google.com>,
"Fangrui Song" <i@maskray.me>,
"Evgenii Stepanov" <eugenis@google.com>,
"Kostya Serebryany" <kcc@google.com>,
"Dmitry Vyukov" <dvyukov@google.com>
Subject: Re: [PATCH] aarch64: Remove ld.so __tls_get_addr plt usage
Date: Mon, 15 Apr 2024 17:22:38 -0300 [thread overview]
Message-ID: <e9fddc2e-b384-4d9f-9204-f2707a4f35de@linaro.org> (raw)
In-Reply-To: <87a5m14odr.fsf@oldenburg.str.redhat.com>
On 10/04/24 05:23, Florian Weimer wrote:
> * Fangrui Song:
>
>> Last time I analyzed the __tls_get_addr interceptor in sanitizers, I
>> have made quite some notes at
>> https://maskray.me/blog/2021-02-14-all-about-thread-local-storage#why-does-compiler-rt-need-to-know-tls-blocks
>>
>> Yes, an interceptor is needed.
>
> There's no guarantuee that TLS access goes through a regular function
> call, so any design that relies on such a call happening is
> fundamentally broken.
>
> Quoting from your article:
>
> | Note: if the allocation is rtld/libc internal and not intercepted,
> | there is no need to unpoison the range. The associated shadow is
> | supposed to be zeros. However, if the allocation is intercepted, the
> | runtime should unpoison the range in case the range reuses a previous
> | allocation which happens to contain poisoned bytes.
> |
> | In glibc, _dl_allocate_tls and _dl_deallocate_tls call malloc/free
> | functions which are internal and not intercepted, so the allocations
> | are opaque to the runtime and the shadow bytes are all zeroes.
>
> I don't think this is accurate. We call the application malloc/free for
> non-main threads after initialization.
>
> Having an accurate description of sanitizer needs in this area would be
> really helpful, but I think we are not quite there yet. (This is
> different from an API description.)
>
> I think there are several aspects here:
>
> (a) Avoid false errors for bounds checks for Address Sanitizer.
>
> (b) Support pointer discovery for Leak Sanitizer (essentially conservative
> garbage collection).
>
> (c) Avoid false data race reports for Thread Sanitizer after TLS reuse
> from one thread for a different thread (only with non-overlapping
> lifetimes).
>
> Based on your description, I'm not sure if (a) is actually a problem.
> If we don't use application malloc for TLS allocations, bounds checking
> is bypassed apparently? And if we use malloc, out-of-bounds accesses
> would be actual bugs.
>
> Aspect (b) is a real issue. Could we address that by allocating the TCB
> (with static TLS) and all dynamic TLS with application malloc (or
> rather, memalign/aligned_alloc), and keep a pointer to the allocation on
> the thread stack? Then a conservative collector could find it, and scan
> it for pointers. A gap remains for the main thread, whose TCB is not
> allocated using application malloc—and can't be, as the application
> malloc itself very likely depends on the TCB already being there. We
> could switch TCBs after allocating another one with malloc, but that
> would require some hand-off protocol, I believe. Maybe it's better to
> register early allocations with the sanitizer directly, using some
> appropriate API.
Using malloc will also improve TCB hardening [1], so I think it would be
valuable to implement regardless of sanitizer work.
>
> For (c), we could just stop caching TCBs after thread exit. If we call
> free, and reallocate for the new thread, that should avoid the false
> data race. This issue does not affect the main thread.
We already have a tunable, glibc.pthread.stack_cache_size, which controls
the thread cached size and setting to 0 should disable it. I do not think
API to dynamically change tunables is a good approach (we might have
potential issues to adapt the code to a dynamic value), so maybe an option
would be to have interposable symbol programs could implement that can
override the tunable values at programs startup.
>
> Based on that, I don't think we need to support discovery of TLS areas,
> or export any other internal implementation details. We just need to
> use more malloc within glibc if we detect an active sanitizer, and find
> a way to make the TCB allocation of the main thread known to the
> sanitizer.
>
> Thanks,
> Florian
>
[1] https://sourceware.org/bugzilla/show_bug.cgi?id=22850
prev parent reply other threads:[~2024-04-15 20:22 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-05 12:35 Adhemerval Zanella
2024-04-05 14:58 ` Szabolcs Nagy
2024-04-05 16:29 ` Adhemerval Zanella Netto
2024-04-06 17:40 ` Szabolcs Nagy
2024-04-08 8:04 ` Florian Weimer
2024-04-07 20:29 ` Cristian Rodríguez
2024-04-08 7:26 ` Szabolcs Nagy
2024-04-08 16:57 ` Adhemerval Zanella Netto
2024-04-09 8:30 ` Szabolcs Nagy
2024-04-09 14:03 ` Adhemerval Zanella Netto
2024-04-09 14:05 ` H.J. Lu
2024-04-09 14:11 ` Palmer Dabbelt
2024-04-09 14:46 ` H.J. Lu
2024-04-09 17:50 ` Fangrui Song
2024-04-10 7:29 ` Szabolcs Nagy
2024-04-10 8:23 ` Florian Weimer
2024-04-10 15:46 ` enh
2024-04-15 11:41 ` Florian Weimer
2024-04-15 20:22 ` Adhemerval Zanella Netto [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=e9fddc2e-b384-4d9f-9204-f2707a4f35de@linaro.org \
--to=adhemerval.zanella@linaro.org \
--cc=cristian@rodriguez.im \
--cc=dvyukov@google.com \
--cc=eugenis@google.com \
--cc=fweimer@redhat.com \
--cc=hjl.tools@gmail.com \
--cc=i@maskray.me \
--cc=kcc@google.com \
--cc=libc-alpha@sourceware.org \
--cc=maskray@google.com \
--cc=szabolcs.nagy@arm.com \
--cc=vitalybuka@google.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).