On 9/30/2023 3:38 AM, Ard Biesheuvel wrote:
On Thu, 28 Sept 2023 at 00:46, Aiqun(Maria) Yu <quic_aiquny@xxxxxxxxxxx> wrote:
On 9/28/2023 8:40 AM, Aiqun(Maria) Yu wrote:
On 9/27/2023 6:11 PM, Ard Biesheuvel wrote:
On Wed, 27 Sept 2023 at 09:44, Aiqun(Maria) Yu
<quic_aiquny@xxxxxxxxxxx> wrote:
On 9/27/2023 5:26 PM, Ard Biesheuvel wrote:
Hello Maria,
On Wed, 27 Sept 2023 at 06:00, Maria Yu <quic_aiquny@xxxxxxxxxxx>
wrote:
Registers r7 is removed in clobber list, so compiler may choose r7 for
local variables usage, while r7 will be actually updated by the
inline asm
code.
The inline asm does not update R7, it preserves and restores it.
That is the asm is updating r7 purposely and compiler still choose r7
for the asm local varialbe and use it inside the asm code.
So the change is to fix the issue when "r7 is removed from the clobber
list of current asm code while actually r7 shouldn't be choosed for the
current asm local variables".
The issue is only reproducible when ftrace is not enabled, and r7 is
removed from the current clobber list.
Let me have the assemble code that will make you understand better.
--the original code:
"mov r11, r7 \n\t"
...
"ldmia %[regs], {r0-r7} \n\t"
"blx %[fn] \n\t"
...
"mov r7, r11 \n\t"
--After compile to choose register for [fn] and [regs].
mov r11, r7
ldr r7, [r1, #16] //r7 used for store asi->insn_fn
...
ldmia.w ip, {r0, r1, r2, r3, r4, r5, r6, r7}
blx r7
...
mov r7,r11
The current change is to avoid by fix the registers for local variable
usage and not choose r7 for [fn].
OK, I understand now, thanks.
Would it help if we just always enabled frame pointers for this source
file?
The compiler(clang 17 here I am using) will still complain for "inline
asm clobber list contains reserved registers" when ftrace enabled.
More over, the local variable have fixed registers was align with
current solution as well. we referenced other asm inlined function
inside kprobe folder, like __kprobes functions inside actions-common.c
files, it is also have similar solution to fixed register usage for
local variables which will be used inside similar inline asm code.
I tested the approach below, and it seems to work fine both with
ftrace enabled and disabled. R7 is no longer selected by the compiler
as an inline asm argument, but is still in charge of the register
allocation.
Thx for the testing.
I have re-test with the clean change only, and it is working same as
your result.
R7 can be reserved when with "-fno-omit-frame-pointer" added as CFLAG
for this file.
The side effect of "-fno-omit-frame-pointer" is that, the additional
instruction is added for those functions in this file, for example:
with "-fno-omit-frame-pointer", instructions will be like:
push {r4, r5, r6, r7, lr}
add r7, sp, #12
stmdb sp!, {r8, r9, sl, fp}
with "-fomit-frame-pointer", instructions will be like:
stmdb sp!, {r4, r5, r6, r7, r8, r9, fp, lr}
My worry is about the unnecessary code size and performance impact.
While myself is not deny the solution of "-fno-omit-frame-pointer",
since in kernel there is other example use the similar solution.
With your approach, the compiler is still free to use R7, and we have
had issues in the past where the compiler does not honour the register
asm ("rX") assignment strictly.
If the compiler does not honour the register strictly, and don't have a
specific rule, it is a problem in existing similar functionality as
well. There is the register asm ("rX") usage across different similar
functions.
example code like emulate_generic_r0_12_noflags() which is used similar
specify fn regist in file actions-common.c:
register void *rregs asm("r1") = regs;
register void *rfn asm("lr") = asi->insn_fn; /**if the compiler
choosing r0-r12, it will be a runtime error. **/
__asm__ __volatile__ (
"stmdb sp!, {%[regs], r11} \n\t"
"ldmia %[regs], {r0-r12} \n\t"
...
"blx %[fn] \n\t"
Could you please explain how you produced the "inline asm clobber list
contains reserved registers" with this change?
I was forget to remove "r7" in the clobber list and test along with this
change and that's why the compiler is complaining "inline asm clobber
list contains reserved registers".
Thx for the correction.
--- a/arch/arm/probes/kprobes/Makefile
+++ b/arch/arm/probes/kprobes/Makefile
@@ -8,6 +8,7 @@ test-kprobes-objs := test-core.o
ifdef CONFIG_THUMB2_KERNEL
obj-$(CONFIG_KPROBES) += actions-thumb.o checkers-thumb.o
+CFLAGS_actions-thumb.o += -fno-omit-frame-pointer
test-kprobes-objs += test-thumb.o
else
obj-$(CONFIG_KPROBES) += actions-arm.o checkers-arm.o
Just come back from long vacation, so delayed response. Thx for the
consideration.
--
Thx and BRs,
Aiqun(Maria) Yu
