[RFC] [PATCH V2, AARCH64]: Re: [RFC] [PATCH, AARCH64] Machine descriptions to support stack smashing protection

[Venkataramanan Kumar <venkataramanan dot kumar at linaro dot org>]

Hi Joseph/Jakub,

Attached is Version 2 patch that adds machine descriptions for stack
protection in Aarch64. I have removed the incorrect test case changes
from the previous patch.

To make GCC compatible with glibc, I have added a test for aarch64 in
"GCC/configure".
This tests for the glibc version >= 2.19, generate TLS based stack
guard access, otherwise will fall back to global variable
__stack_chk_guard based access.

Also I will be submitting a glibc patch to export __stack_chk_guard
and initialize it with proper value, to make sure that it coexists
with TLS based stack guard access for aarch64.

I have posted code snippet here.
http://gcc.gnu.org/ml/gcc-patches/2013-11/msg02968.html

ChangeLog:

2013-11-26  Venkataramanan Kumar  <venkataramanan.kumar@linaro.org>
        * configure.ac (gcc_cv_libc_provides_tls_ssp): Add test to
        check TLS support in target C library for Aarch64.
        * configure: Regenerate.
        * config.in: Regenerate.
        * config/aarch64/aarch64.md (stack_protect_set, stack_protect_test)
        (stack_protect_set_<mode>, stack_protect_test_<mode>): Add
        initial machine description for Stack Smashing Protector.
        * config/aarch64/aarch64-linux.h (TARGET_THREAD_SSP_OFFSET): Define.

2013-11-26  Venkataramanan Kumar  <venkataramanan.kumar@linaro.org>
        * g++.dg/fstack-protector-strong.C: Add aarch64 target.
        * gcc.dg/fstack-protector-strong.c: Likewise.

Let me know your feed back and please advice on improving it further.

regards,
Venkat.

On 23 November 2013 09:32, Venkataramanan Kumar
<venkataramanan.kumar@linaro.org> wrote:
> Hi Joseph,
>
> Thank you for the detail explanation.
>
>> You need to ensure that, when new glibc is built, whatever compiler it's
>> built with, it continues to export the __stack_chk_guard symbol at version
>> GLIBC_2.17.  Furthermore, if any released GCC version would generate
>> references to __stack_chk_guard when compiling code for AArch64 with stack
>> protection, you need to ensure __stack_chk_guard is a normal symbol not a
>> compat symbol so that people can continue to link against new glibc when
>> using old GCC.  If it's only a limited range of development versions of
>> GCC that could have generated code using __stack_chk_guard because
>> released versions didn't support stack protection on AArch64 at all, a
>> compat symbol would probably be OK (but you should still ensure that the
>> variable gets initialized with the correct value for any applications
>> built with those development versions of GCC).
>
> As you said when THREAD_SET_STACK_GUARD is set glibc does not export
> __stack_chk_guard. So I am planning to change the export logic by
> adding a new macro EXPORT_GLOBAL_STACK_GUARD
> and set it for Aarch64 port in glibc.
>
> ----snip----
> --- a/csu/libc-start.c
> +++ b/csu/libc-start.c
> -# ifndef THREAD_SET_STACK_GUARD
> +
> +#if !defined(THREAD_SET_STACK_GUARD) || defined(EXPORT_GLOBAL_STACK_GUARD)
>  /* Only exported for architectures that don't store the stack guard canary
>     in thread local area.  */
>  uintptr_t __stack_chk_guard attribute_relro;
> -# endif
> +#endif
> +
> ----snip----
>
> I will find a better way to version that symbol as well. I will sent a
> RFC patch to glibc mailing list.
>
> On the GCC side, trunk GCC configure script checks and sets
> TARGET_LIBC_PROVIDES_SSP support when glibc is >=2.4
>
> -----snip----
> # Test for stack protector support in target C library.
> { $as_echo "$as_me:${as_lineno-$LINENO}: checking __stack_chk_fail in
> target C library" >&5
> $as_echo_n "checking __stack_chk_fail in target C library... " >&6; }
> if test "${gcc_cv_libc_provides_ssp+set}" = set; then :
>   $as_echo_n "(cached) " >&6
> else
>   gcc_cv_libc_provides_ssp=no
>     case "$target" in
>        *-*-linux* | *-*-kfreebsd*-gnu | *-*-knetbsd*-gnu)
>       # glibc 2.4 and later provides __stack_chk_fail and
>       # either __stack_chk_guard, or TLS access to stack guard canary.
>
> if test $glibc_version_major -gt 2 \
>   || ( test $glibc_version_major -eq 2 && test $glibc_version_minor
> -ge 4 ); then :
>   gcc_cv_libc_provides_ssp=yes
>
>
> if test x$gcc_cv_libc_provides_ssp = xyes; then
>
> $as_echo "#define TARGET_LIBC_PROVIDES_SSP 1" >>confdefs.h
> fi
> ----snip-----
>
> To make GCC for AArch64 generate TLS based stack access for glibc >=
> 2.19 I need to introduce a new macro
> TARGET_LIBC_PROVIDES_TLS_SSP and check and set it for glibc >= 2.19 in
> GCC configure .
>
> Any better approach to this since it is specific to Aarch64?
>
> regards,
> Venkat.
>
> On 20 November 2013 22:38, Joseph S. Myers <joseph@codesourcery.com> wrote:
>> On Wed, 20 Nov 2013, Venkataramanan Kumar wrote:
>>
>>> > I would like to see a clear description of what happens with all eight
>>> > combinations of (glibc version does or doesn't support this, GCC building
>>> > glibc does or doesn't support this, GCC building user program does or
>>> > doesn't support this).  Which of the (GCC building glibc, glibc)
>>> > combinations will successfully build glibc?  Will all such glibcs be
>>> > binary-compatible?  Will both old and new GCC work for building user
>>> > programs with both old and new glibc?
>>>
>>> Can you please clarify why we need to consider "the gcc compiler that
>>> builds the glibc" in the combinations you want me to describe. I am
>>> not able to understand that.
>>
>> Let's imagine this support goes in GCC 4.9 and the glibc support goes in
>> glibc 2.19, whereas GCC 4.8 and glibc 2.18 are versions without this
>> support.
>>
>> * Building glibc 2.18 with GCC 4.8 already works (I presume).
>>
>> * Suppose you use GCC 4.9 to build glibc 2.18.  Does this work?  If it
>> works, it must produce a glibc binary that's ABI compatible with one built
>> with GCC 4.8, meaning same symbols exported at same symbol versions.
>>
>> * Suppose you build glibc 2.19 with GCC 4.8.  Does this work?  If it does,
>> then it must be ABI compatible with 2.18 (meaning the symbols exported at
>> GLIBC_2.18 or earlier versions must be exactly the same as exported at
>> those versions in 2.18).
>>
>> * Suppose you build glibc 2.19 with GCC 4.9.  This needs to work and must
>> again produce a binary compatible with the previous ones.
>>
>> Note: there is no current glibc support for architectures that gained
>> TLS-based stack guards after 2.4 (meaning they need both a TLS-based
>> scheme and backwards compatibility for binaries using __stack_chk_guard),
>> and your glibc patch doesn't seem to add any.  It looks to me like your
>> glibc patch would have removed the __stack_chk_guard symbol and so failed
>> ABI tests (this symbol being defined only if THREAD_SET_STACK_GUARD isn't
>> defined) - you didn't make clear if your patch was tested with the glibc
>> testsuite including passing the ABI tests.  The normal presumption is that
>> it's not acceptable to remove exported symbols from glibc as some
>> application might be using them.
>>
>> You need to ensure that, when new glibc is built, whatever compiler it's
>> built with, it continues to export the __stack_chk_guard symbol at version
>> GLIBC_2.17.  Furthermore, if any released GCC version would generate
>> references to __stack_chk_guard when compiling code for AArch64 with stack
>> protection, you need to ensure __stack_chk_guard is a normal symbol not a
>> compat symbol so that people can continue to link against new glibc when
>> using old GCC.  If it's only a limited range of development versions of
>> GCC that could have generated code using __stack_chk_guard because
>> released versions didn't support stack protection on AArch64 at all, a
>> compat symbol would probably be OK (but you should still ensure that the
>> variable gets initialized with the correct value for any applications
>> built with those development versions of GCC).
>>
>> --
>> Joseph S. Myers
>> joseph@codesourcery.com

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