[PATCH] Correct PowerPC VDSO call frame info

Alan Modra amodra at gmail.com
Fri Sep 14 09:27:04 AEST 2018


There is control flow in __kernel_clock_gettime that reaches label 99
without saving lr in r12.  CFI info however is interpreted by the
unwinder without reference to control flow: It's a simple matter of
"Execute all the CFI opcodes up to the current address".  That means
the unwinder thinks r12 contains the return address at label 99.
Disabuse it of that notion by resetting CFI for the return address at
label 99.

Note that the ".cfi_restore lr" could have gone anywhere from the
"mtlr r12" a few instructions earlier to the instruction at label 99.
I put the CFI as late as possible, because in general that's best
practice (and if possible grouped with other CFI in order to reduce
the number of CFI opcodes executed when unwinding).  Using r12 as the
return address is perfectly fine after the "mtlr r12" since r12 on
that code path still contains the return address.

__get_datapage also has a CFI error.  That function temporarily saves
lr in r0, and reflects that fact with ".cfi_register lr,r0".  A later
use of r0 means the CFI at that point isn't correct, as r0 no longer
contains the return address.  Fix that too.

Signed-off-by: Alan Modra <amodra at gmail.com>

diff --git a/arch/powerpc/kernel/vdso32/datapage.S b/arch/powerpc/kernel/vdso32/datapage.S
index 3745113fcc65..2a7eb5452aba 100644
--- a/arch/powerpc/kernel/vdso32/datapage.S
+++ b/arch/powerpc/kernel/vdso32/datapage.S
@@ -37,6 +37,7 @@ data_page_branch:
 	mtlr	r0
 	addi	r3, r3, __kernel_datapage_offset-data_page_branch
 	lwz	r0,0(r3)
+  .cfi_restore lr
 	add	r3,r0,r3
 	blr
   .cfi_endproc
diff --git a/arch/powerpc/kernel/vdso32/gettimeofday.S b/arch/powerpc/kernel/vdso32/gettimeofday.S
index 769c2624e0a6..1e0bc5955a40 100644
--- a/arch/powerpc/kernel/vdso32/gettimeofday.S
+++ b/arch/powerpc/kernel/vdso32/gettimeofday.S
@@ -139,6 +139,7 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
 	 */
 99:
 	li	r0,__NR_clock_gettime
+  .cfi_restore lr
 	sc
 	blr
   .cfi_endproc
diff --git a/arch/powerpc/kernel/vdso64/datapage.S b/arch/powerpc/kernel/vdso64/datapage.S
index abf17feffe40..bf9668691511 100644
--- a/arch/powerpc/kernel/vdso64/datapage.S
+++ b/arch/powerpc/kernel/vdso64/datapage.S
@@ -37,6 +37,7 @@ data_page_branch:
 	mtlr	r0
 	addi	r3, r3, __kernel_datapage_offset-data_page_branch
 	lwz	r0,0(r3)
+  .cfi_restore lr
 	add	r3,r0,r3
 	blr
   .cfi_endproc
diff --git a/arch/powerpc/kernel/vdso64/gettimeofday.S b/arch/powerpc/kernel/vdso64/gettimeofday.S
index c002adcc694c..a4ed9edfd5f0 100644
--- a/arch/powerpc/kernel/vdso64/gettimeofday.S
+++ b/arch/powerpc/kernel/vdso64/gettimeofday.S
@@ -169,6 +169,7 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
 	 */
 99:
 	li	r0,__NR_clock_gettime
+  .cfi_restore lr
 	sc
 	blr
   .cfi_endproc

-- 
Alan Modra
Australia Development Lab, IBM


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