Re: [PATCH v2] MIPS: Avoid VDSO ABI breakage due to global register variable

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

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Hi Paul,

Happy new year.

On 02/01/2020 04:50, Paul Burton wrote:
> Declaring __current_thread_info as a global register variable has the
> effect of preventing GCC from saving & restoring its value in cases
> where the ABI would typically do so.
> 
> To quote GCC documentation:
> 
>> If the register is a call-saved register, call ABI is affected: the
>> register will not be restored in function epilogue sequences after the
>> variable has been assigned. Therefore, functions cannot safely return
>> to callers that assume standard ABI.
> 
> When our position independent VDSO is built for the n32 or n64 ABIs all
> functions it exposes should be preserving the value of $gp/$28 for their
> caller, but in the presence of the __current_thread_info global register
> variable GCC stops doing so & simply clobbers $gp/$28 when calculating
> the address of the GOT.
> 
> In cases where the VDSO returns success this problem will typically be
> masked by the caller in libc returning & restoring $gp/$28 itself, but
> that is by no means guaranteed. In cases where the VDSO returns an error
> libc will typically contain a fallback path which will now fail
> (typically with a bad memory access) if it attempts anything which
> relies upon the value of $gp/$28 - eg. accessing anything via the GOT.
> 

First of all good catch. I just came back from holidays and seems that you guys
had fun without me :)

Did you consider the option to use "-ffixed-gp -ffixed-28" as a compilation
flags for the vDSO library (Arvind was mentioning it as well in his reply)?
According to the GCC manual "treats the register as a fixed register; generated
code should never refer to it"
(https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options)

I did some experiments this morning an seems that on MIPS the convention is not
honored at least on GCC-9. Do you know who to contact to get it enabled/fixed in
the compiler?

With this:

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

> One fix for this would be to move the declaration of
> __current_thread_info inside the current_thread_info() function,
> demoting it from global register variable to local register variable &
> avoiding inadvertently creating a non-standard calling ABI for the VDSO.
> Unfortunately this causes issues for clang, which doesn't support local
> register variables as pointed out by commit fe92da0f355e ("MIPS: Changed
> current_thread_info() to an equivalent supported by both clang and GCC")
> which introduced the global register variable before we had a VDSO to
> worry about.
> 
> Instead, fix this by continuing to use the global register variable for
> the kernel proper but declare __current_thread_info as a simple extern
> variable when building the VDSO. It should never be referenced, and will
> cause a link error if it is. This resolves the calling convention issue
> for the VDSO without having any impact upon the build of the kernel
> itself for either clang or gcc.
> 
> Signed-off-by: Paul Burton <paulburton@kernel.org>
> Reported-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
> Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO")
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Christian Brauner <christian.brauner@canonical.com>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: <stable@vger.kernel.org> # v4.4+
> ---
> Changes in v2:
> - Switch to the #ifdef __VDSO__ approach rather than using a local
>   register variable which clang doesn't support.
> ---
>  arch/mips/include/asm/thread_info.h | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/include/asm/thread_info.h b/arch/mips/include/asm/thread_info.h
> index 4993db40482c..ee26f9a4575d 100644
> --- a/arch/mips/include/asm/thread_info.h
> +++ b/arch/mips/include/asm/thread_info.h
> @@ -49,8 +49,26 @@ struct thread_info {
>  	.addr_limit	= KERNEL_DS,		\
>  }
>  
> -/* How to get the thread information struct from C.  */
> +/*
> + * A pointer to the struct thread_info for the currently executing thread is
> + * held in register $28/$gp.
> + *
> + * We declare __current_thread_info as a global register variable rather than a
> + * local register variable within current_thread_info() because clang doesn't
> + * support explicit local register variables.
> + *
> + * When building the VDSO we take care not to declare the global register
> + * variable because this causes GCC to not preserve the value of $28/$gp in
> + * functions that change its value (which is common in the PIC VDSO when
> + * accessing the GOT). Since the VDSO shouldn't be accessing
> + * __current_thread_info anyway we declare it extern in order to cause a link
> + * failure if it's referenced.
> + */
> +#ifdef __VDSO__
> +extern struct thread_info *__current_thread_info;
> +#else
>  register struct thread_info *__current_thread_info __asm__("$28");
> +#endif
>  
>  static inline struct thread_info *current_thread_info(void)
>  {
> 

-- 
Regards,
Vincenzo

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