Messages in this thread Patch in this message | | | From | Paul Burton <> | Subject | [PATCH] MIPS: Don't declare __current_thread_info globally | Date | Wed, 1 Jan 2020 09:59:16 -0800 |
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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.
Fix this by moving 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.
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+ --- arch/mips/include/asm/thread_info.h | 4 ++-- arch/mips/kernel/relocate.c | 1 + 2 files changed, 3 insertions(+), 2 deletions(-)
diff --git a/arch/mips/include/asm/thread_info.h b/arch/mips/include/asm/thread_info.h index 4993db40482c..aceefc3f9a1a 100644 --- a/arch/mips/include/asm/thread_info.h +++ b/arch/mips/include/asm/thread_info.h @@ -50,10 +50,10 @@ struct thread_info { } /* How to get the thread information struct from C. */ -register struct thread_info *__current_thread_info __asm__("$28"); - static inline struct thread_info *current_thread_info(void) { + register struct thread_info *__current_thread_info __asm__("$28"); + return __current_thread_info; } diff --git a/arch/mips/kernel/relocate.c b/arch/mips/kernel/relocate.c index 3d80a51256de..c9afdc39b003 100644 --- a/arch/mips/kernel/relocate.c +++ b/arch/mips/kernel/relocate.c @@ -296,6 +296,7 @@ static inline int __init relocation_addr_valid(void *loc_new) void *__init relocate_kernel(void) { + register struct thread_info *__current_thread_info __asm__("$28"); void *loc_new; unsigned long kernel_length; unsigned long bss_length; -- 2.24.0
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