Hello Maria,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.
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.
This caused the runtime behavior wrong.
Could you explain how, exactly? In which cases is the preserve/restore
of R7 failing to achieve the intended result?
R7 needed to be prepared for use when blx to the final fn for kprobe functional as my understanding.
While those kind of reserved registers cannot be set to clobber list
because of error like "inline asm clobber list contains reserved
registers".
To both working for reserved register case and non-reserved register case,
explicitly assign register for local variables which will be used as asm
input.
If we make this change, could we remove the references to R7 altogether?
We have encounter the runtime issue when compiler choose r7 for store "asi->insn_fn" here for example above.
Fixes: dd12e97f3c72 ("ARM: kprobes: treat R7 as the frame pointer register in Thumb2 builds")
Signed-off-by: Maria Yu <quic_aiquny@xxxxxxxxxxx>
---
arch/arm/probes/kprobes/actions-thumb.c | 32 ++++++++++++++++---------
1 file changed, 21 insertions(+), 11 deletions(-)
diff --git a/arch/arm/probes/kprobes/actions-thumb.c b/arch/arm/probes/kprobes/actions-thumb.c
index 51624fc263fc..f667b2f00b3e 100644
--- a/arch/arm/probes/kprobes/actions-thumb.c
+++ b/arch/arm/probes/kprobes/actions-thumb.c
@@ -442,8 +442,10 @@ static unsigned long __kprobes
t16_emulate_loregs(probes_opcode_t insn,
struct arch_probes_insn *asi, struct pt_regs *regs)
{
- unsigned long oldcpsr = regs->ARM_cpsr;
- unsigned long newcpsr;
+ register unsigned long oldcpsr asm("r8") = regs->ARM_cpsr;
+ register unsigned long newcpsr asm("r9");
+ register void *rregs asm("r10") = regs;
+ register void *rfn asm("lr") = asi->insn_fn;
__asm__ __volatile__ (
"msr cpsr_fs, %[oldcpsr] \n\t"
@@ -454,10 +456,10 @@ t16_emulate_loregs(probes_opcode_t insn,
"mov r7, r11 \n\t"
"mrs %[newcpsr], cpsr \n\t"
: [newcpsr] "=r" (newcpsr)
- : [oldcpsr] "r" (oldcpsr), [regs] "r" (regs),
- [fn] "r" (asi->insn_fn)
+ : [oldcpsr] "r" (oldcpsr), [regs] "r" (rregs),
+ [fn] "r" (rfn)
: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r11",
- "lr", "memory", "cc"
+ "memory", "cc"
);
return (oldcpsr & ~APSR_MASK) | (newcpsr & APSR_MASK);
@@ -525,6 +527,9 @@ static void __kprobes
t16_emulate_push(probes_opcode_t insn,
struct arch_probes_insn *asi, struct pt_regs *regs)
{
+ register void *rfn asm("lr") = asi->insn_fn;
+ register void *rregs asm("r10") = regs;
+
__asm__ __volatile__ (
"mov r11, r7 \n\t"
"ldr r9, [%[regs], #13*4] \n\t"
@@ -534,9 +539,9 @@ t16_emulate_push(probes_opcode_t insn,
"str r9, [%[regs], #13*4] \n\t"
"mov r7, r11 \n\t"
:
- : [regs] "r" (regs), [fn] "r" (asi->insn_fn)
+ : [regs] "r" (rregs), [fn] "r" (rfn)
: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r8", "r9", "r11",
- "lr", "memory", "cc"
+ "memory", "cc"
);
}
@@ -561,6 +566,9 @@ static void __kprobes
t16_emulate_pop_nopc(probes_opcode_t insn,
struct arch_probes_insn *asi, struct pt_regs *regs)
{
+ register void *rfn asm("lr") = asi->insn_fn;
+ register void *rregs asm("r8") = regs;
+
__asm__ __volatile__ (
"mov r11, r7 \n\t"
"ldr r9, [%[regs], #13*4] \n\t"
@@ -570,9 +578,9 @@ t16_emulate_pop_nopc(probes_opcode_t insn,
"str r9, [%[regs], #13*4] \n\t"
"mov r7, r11 \n\t"
:
- : [regs] "r" (regs), [fn] "r" (asi->insn_fn)
+ : [regs] "r" (rregs), [fn] "r" (rfn)
: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r9", "r11",
- "lr", "memory", "cc"
+ "memory", "cc"
);
}
@@ -581,6 +589,8 @@ t16_emulate_pop_pc(probes_opcode_t insn,
struct arch_probes_insn *asi, struct pt_regs *regs)
{
register unsigned long pc asm("r8");
+ register void *rfn asm("lr") = asi->insn_fn;
+ register void *rregs asm("r10") = regs;
__asm__ __volatile__ (
"mov r11, r7 \n\t"
@@ -591,9 +601,9 @@ t16_emulate_pop_pc(probes_opcode_t insn,
"str r9, [%[regs], #13*4] \n\t"
"mov r7, r11 \n\t"
: "=r" (pc)
- : [regs] "r" (regs), [fn] "r" (asi->insn_fn)
+ : [regs] "r" (rregs), [fn] "r" (rfn)
: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r9", "r11",
- "lr", "memory", "cc"
+ "memory", "cc"
);
bx_write_pc(pc, regs);
base-commit: 6465e260f48790807eef06b583b38ca9789b6072
--
2.17.1