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objtool: Handle GCC stack pointer adjustment bug

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Arnd Bergmann reported the following warning with GCC 7.1.1:

  fs/fs_pin.o: warning: objtool: pin_kill()+0x139: stack state mismatch: cfa1=7+88 cfa2=7+96

And the kbuild robot reported the following warnings with GCC 5.4.1:

  fs/fs_pin.o: warning: objtool: pin_kill()+0x182: return with modified stack frame
  fs/quota/dquot.o: warning: objtool: dquot_alloc_inode()+0x140: stack state mismatch: cfa1=7+120 cfa2=7+128
  fs/quota/dquot.o: warning: objtool: dquot_free_inode()+0x11a: stack state mismatch: cfa1=7+112 cfa2=7+120

Those warnings are caused by an unusual GCC non-optimization where it
uses an intermediate register to adjust the stack pointer.  It does:

  lea    0x8(%rsp), %rcx
  ...
  mov    %rcx, %rsp

Instead of the obvious:

  add    $0x8, %rsp

It makes no sense to use an intermediate register, so I opened a GCC bug
to track it:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81813

But it's not exactly a high-priority bug and it looks like we'll be
stuck with this issue for a while.  So for now we have to track register
values when they're loaded with stack pointer offsets.

This is kind of a big workaround for a tiny problem, but c'est la vie.
I hope to eventually create a GCC plugin to implement a big chunk of
objtool's functionality.  Hopefully at that point we'll be able to
remove of a lot of these GCC-isms from the objtool code.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/6a41a96884c725e7f05413bb7df40cfe824b2444.1504028945.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Josh Poimboeuf 2017-08-29 12:51:03 -05:00 committed by Ingo Molnar
parent 499934898f
commit dd88a0a0c8
4 changed files with 89 additions and 91 deletions
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102
tools/objtool/arch/x86/decode.c
View file

@ -86,8 +86,8 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
struct insn insn;
int x86_64, sign;
unsigned char op1, op2, rex = 0, rex_b = 0, rex_r = 0, rex_w = 0,
modrm = 0, modrm_mod = 0, modrm_rm = 0, modrm_reg = 0,
sib = 0;
rex_x = 0, modrm = 0, modrm_mod = 0, modrm_rm = 0,
modrm_reg = 0, sib = 0;
x86_64 = is_x86_64(elf);
if (x86_64 == -1)
@ -114,6 +114,7 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
rex = insn.rex_prefix.bytes[0];
rex_w = X86_REX_W(rex) >> 3;
rex_r = X86_REX_R(rex) >> 2;
rex_x = X86_REX_X(rex) >> 1;
rex_b = X86_REX_B(rex);
}
@ -217,6 +218,18 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
op->dest.reg = CFI_BP;
break;
}
if (rex_w && !rex_b && modrm_mod == 3 && modrm_rm == 4) {
/* mov reg, %rsp */
*type = INSN_STACK;
op->src.type = OP_SRC_REG;
op->src.reg = op_to_cfi_reg[modrm_reg][rex_r];
op->dest.type = OP_DEST_REG;
op->dest.reg = CFI_SP;
break;
}
/* fallthrough */
case 0x88:
if (!rex_b &&
@ -269,7 +282,17 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
break;
case 0x8d:
if (rex == 0x48 && modrm == 0x65) {
if (sib == 0x24 && rex_w && !rex_b && !rex_x) {
/* lea disp(%rsp), reg */
*type = INSN_STACK;
op->src.type = OP_SRC_ADD;
op->src.reg = CFI_SP;
op->src.offset = insn.displacement.value;
op->dest.type = OP_DEST_REG;
op->dest.reg = op_to_cfi_reg[modrm_reg][rex_r];
} else if (rex == 0x48 && modrm == 0x65) {
/* lea disp(%rbp), %rsp */
*type = INSN_STACK;
@ -278,71 +301,9 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
op->src.offset = insn.displacement.value;
op->dest.type = OP_DEST_REG;
op->dest.reg = CFI_SP;
break;
}
if (rex == 0x48 && (modrm == 0xa4 || modrm == 0x64) &&
sib == 0x24) {
/* lea disp(%rsp), %rsp */
*type = INSN_STACK;
op->src.type = OP_SRC_ADD;
op->src.reg = CFI_SP;
op->src.offset = insn.displacement.value;
op->dest.type = OP_DEST_REG;
op->dest.reg = CFI_SP;
break;
}
if (rex == 0x48 && modrm == 0x2c && sib == 0x24) {
/* lea (%rsp), %rbp */
*type = INSN_STACK;
op->src.type = OP_SRC_REG;
op->src.reg = CFI_SP;
op->dest.type = OP_DEST_REG;
op->dest.reg = CFI_BP;
break;
}
if (rex == 0x4c && modrm == 0x54 && sib == 0x24 &&
insn.displacement.value == 8) {
/*
* lea 0x8(%rsp), %r10
*
* Here r10 is the "drap" pointer, used as a stack
* pointer helper when the stack gets realigned.
*/
*type = INSN_STACK;
op->src.type = OP_SRC_ADD;
op->src.reg = CFI_SP;
op->src.offset = 8;
op->dest.type = OP_DEST_REG;
op->dest.reg = CFI_R10;
break;
}
if (rex == 0x4c && modrm == 0x6c && sib == 0x24 &&
insn.displacement.value == 16) {
/*
* lea 0x10(%rsp), %r13
*
* Here r13 is the "drap" pointer, used as a stack
* pointer helper when the stack gets realigned.
*/
*type = INSN_STACK;
op->src.type = OP_SRC_ADD;
op->src.reg = CFI_SP;
op->src.offset = 16;
op->dest.type = OP_DEST_REG;
op->dest.reg = CFI_R13;
break;
}
if (rex == 0x49 && modrm == 0x62 &&
insn.displacement.value == -8) {
} else if (rex == 0x49 && modrm == 0x62 &&
insn.displacement.value == -8) {
/*
* lea -0x8(%r10), %rsp
@ -356,11 +317,9 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
op->src.offset = -8;
op->dest.type = OP_DEST_REG;
op->dest.reg = CFI_SP;
break;
}
if (rex == 0x49 && modrm == 0x65 &&
insn.displacement.value == -16) {
} else if (rex == 0x49 && modrm == 0x65 &&
insn.displacement.value == -16) {
/*
* lea -0x10(%r13), %rsp
@ -374,7 +333,6 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
op->src.offset = -16;
op->dest.type = OP_DEST_REG;
op->dest.reg = CFI_SP;
break;
}
break;

2
tools/objtool/cfi.h
View file

@ -40,7 +40,7 @@
#define CFI_R14 14
#define CFI_R15 15
#define CFI_RA 16
#define CFI_NUM_REGS 17
#define CFI_NUM_REGS 17
struct cfi_reg {
int base;

75
tools/objtool/check.c
View file

@ -218,8 +218,10 @@ static void clear_insn_state(struct insn_state *state)
memset(state, 0, sizeof(*state));
state->cfa.base = CFI_UNDEFINED;
for (i = 0; i < CFI_NUM_REGS; i++)
for (i = 0; i < CFI_NUM_REGS; i++) {
state->regs[i].base = CFI_UNDEFINED;
state->vals[i].base = CFI_UNDEFINED;
}
state->drap_reg = CFI_UNDEFINED;
state->drap_offset = -1;
}
@ -1201,24 +1203,47 @@ static int update_insn_state(struct instruction *insn, struct insn_state *state)
switch (op->src.type) {
case OP_SRC_REG:
if (cfa->base == op->src.reg && cfa->base == CFI_SP &&
op->dest.reg == CFI_BP && regs[CFI_BP].base == CFI_CFA &&
regs[CFI_BP].offset == -cfa->offset) {
if (op->src.reg == CFI_SP && op->dest.reg == CFI_BP) {
/* mov %rsp, %rbp */
cfa->base = op->dest.reg;
state->bp_scratch = false;
} else if (state->drap) {
if (cfa->base == CFI_SP &&
regs[CFI_BP].base == CFI_CFA &&
regs[CFI_BP].offset == -cfa->offset) {
/* drap: mov %rsp, %rbp */
regs[CFI_BP].base = CFI_BP;
regs[CFI_BP].offset = -state->stack_size;
state->bp_scratch = false;
} else if (!no_fp) {
/* mov %rsp, %rbp */
cfa->base = op->dest.reg;
state->bp_scratch = false;
}
WARN_FUNC("unknown stack-related register move",
insn->sec, insn->offset);
return -1;
else if (state->drap) {
/* drap: mov %rsp, %rbp */
regs[CFI_BP].base = CFI_BP;
regs[CFI_BP].offset = -state->stack_size;
state->bp_scratch = false;
}
}
else if (op->dest.reg == cfa->base) {
/* mov %reg, %rsp */
if (cfa->base == CFI_SP &&
state->vals[op->src.reg].base == CFI_CFA) {
/*
* This is needed for the rare case
* where GCC does something dumb like:
*
* lea 0x8(%rsp), %rcx
* ...
* mov %rcx, %rsp
*/
cfa->offset = -state->vals[op->src.reg].offset;
state->stack_size = cfa->offset;
} else {
cfa->base = CFI_UNDEFINED;
cfa->offset = 0;
}
}
break;
@ -1240,11 +1265,25 @@ static int update_insn_state(struct instruction *insn, struct insn_state *state)
break;
}
if (op->dest.reg != CFI_BP && op->src.reg == CFI_SP &&
cfa->base == CFI_SP) {
if (op->src.reg == CFI_SP && cfa->base == CFI_SP) {
/* drap: lea disp(%rsp), %drap */
state->drap_reg = op->dest.reg;
/*
* lea disp(%rsp), %reg
*
* This is needed for the rare case where GCC
* does something dumb like:
*
* lea 0x8(%rsp), %rcx
* ...
* mov %rcx, %rsp
*/
state->vals[op->dest.reg].base = CFI_CFA;
state->vals[op->dest.reg].offset = \
-state->stack_size + op->src.offset;
break;
}

1
tools/objtool/check.h
View file

@ -33,6 +33,7 @@ struct insn_state {
bool bp_scratch;
bool drap;
int drap_reg, drap_offset;
struct cfi_reg vals[CFI_NUM_REGS];
};
struct instruction {