[PATCH] elf: Add __libc_get_static_tls_bounds [BZ #16291]

Florian Weimer fweimer@redhat.com
Mon Nov 29 19:30:44 GMT 2021


* Fāng-ruì Sòng:

> PING^3

I think the core issue with this patch is like this:

* I do not want to commit glibc to a public API that disallows future
  changes to the way we allocate static TLS.  While static TLS objects
  cannot move in memory, the extent of the static TLS area (minimum and
  maximum address) is not fixed by ABI necessities.

* Joseph does not want to add a GLIBC_PRIVATE ABI that is exclusively
  used externally.

I have tried repeatly to wrap my head around how the sanitizers use the
static TLS boundary information.  Based on what I can tell, there are
several applications:

1. Thead Sanitizer needs to know application-accessible thread-specific
   memory that is carried via the glibc thread (stack) cache from one
   thread to another one, seemingly without synchronization.  Since the
   synchronization happens internally within glibc, without that extra
   knowledge, Thread Sanitizer would report a false positive.  This
   covers only data to which the application has direct access, internal
   access by glibc does not count because it is not going to be
   instrumented.

2. Address Sanitizer needs TLS boundary information for bounds checking.
   Again this only applies to accesses that can be instrumented, so only
   objects whose addresses leak to application code count.  (Maybe this
   is a fringe use case, though: it seems to apply only to “extern
   __thread int a[];“ and similar declarations, where the declared type
   is not enough.)

3. Leak Sanitizer needs to find all per-thread pointers pointing into
   the malloc heap, within memory not itself allocated by malloc.  This
   includes the DTV, pthread_getspecific metadata, and perhaps also user
   data set by pthread_getspecific, and of course static TLS variables.
   This goes someone beyond what people would usually consider static
   TLS: the DTV and struct pthread are not really part of static TLS as
   such.  And struct pthread has several members that contain malloc'ed
   pointers.

Is this a complete list of uses?

I *think* you can get what you need via existing GLIBC_PRIVATE
interfaces.  But in order to describe how to caox the data out of glibc,
I need to know what you need.

(Cc:ing a few people from a recent GCC thread.)

Thanks,
Florian



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