ARM: cacheflush: avoid clobbering the frame pointer

Thumb2 uses R7 rather than R11 as the frame pointer, and even if we
rarely use a frame pointer to begin with when building in Thumb2 mode,
there are cases where it is required by the compiler (Clang when
inserting profiling hooks via -pg)

However, preserving and restoring the frame pointer is risky, as any
unhandled exceptions raised in the mean time will produce a bogus
backtrace, and it would be better not to touch the frame pointer at all.
This is the case even when CONFIG_FRAME_POINTER is not set, as the
unwind directive used by the unwinder may also use R7 or R11 as the
unwind anchor, even if the frame pointer is not managed strictly
according to the frame pointer ABI.

So let's tweak the cacheflush asm code not to clobber R7 or R11 at all,
so that we can drop R7 from the clobber lists of the inline asm blocks
that call these routines, and remove the code that preserves/restores
R11.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

1 files changed, 3 insertions, 9 deletions

diff --git a/arch/arm/include/asm/cacheflush.h b/arch/arm/include/asm/cacheflush.h
index e68fb879e4f9..d27782331556 100644
--- a/arch/arm/include/asm/cacheflush.h
+++ b/arch/arm/include/asm/cacheflush.h
@@ -446,15 +446,10 @@ static inline void __sync_cache_range_r(volatile void *p, size_t size)
  *   however some exceptions may exist.  Caveat emptor.
  *
  * - The clobber list is dictated by the call to v7_flush_dcache_*.
- *   fp is preserved to the stack explicitly prior disabling the cache
- *   since adding it to the clobber list is incompatible with having
- *   CONFIG_FRAME_POINTER=y.  ip is saved as well if ever r12-clobbering
- *   trampoline are inserted by the linker and to keep sp 64-bit aligned.
  */
 #define v7_exit_coherency_flush(level) \
 	asm volatile( \
 	".arch	armv7-a \n\t" \
-	"stmfd	sp!, {fp, ip} \n\t" \
 	"mrc	p15, 0, r0, c1, c0, 0	@ get SCTLR \n\t" \
 	"bic	r0, r0, #"__stringify(CR_C)" \n\t" \
 	"mcr	p15, 0, r0, c1, c0, 0	@ set SCTLR \n\t" \
@@ -464,10 +459,9 @@ static inline void __sync_cache_range_r(volatile void *p, size_t size)
 	"bic	r0, r0, #(1 << 6)	@ disable local coherency \n\t" \
 	"mcr	p15, 0, r0, c1, c0, 1	@ set ACTLR \n\t" \
 	"isb	\n\t" \
-	"dsb	\n\t" \
-	"ldmfd	sp!, {fp, ip}" \
-	: : : "r0","r1","r2","r3","r4","r5","r6","r7", \
-	      "r9","r10","lr","memory" )
+	"dsb" \
+	: : : "r0","r1","r2","r3","r4","r5","r6", \
+	      "r9","r10","ip","lr","memory" )
 
 void flush_uprobe_xol_access(struct page *page, unsigned long uaddr,
 			     void *kaddr, unsigned long len);