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SubjectRe: Getting empty callchain from perf_callchain_kernel()
On Fri, May 24, 2019 at 10:20:52AM +0800, Kairui Song wrote:
> On Fri, May 24, 2019 at 1:27 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > On Fri, May 24, 2019 at 12:41:59AM +0800, Kairui Song wrote:
> > > On Thu, May 23, 2019 at 11:24 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > > >
> > > > On Thu, May 23, 2019 at 10:50:24PM +0800, Kairui Song wrote:
> > > > > > > Hi Josh, this still won't fix the problem.
> > > > > > >
> > > > > > > Problem is not (or not only) with ___bpf_prog_run, what actually went
> > > > > > > wrong is with the JITed bpf code.
> > > > > >
> > > > > > There seem to be a bunch of issues. My patch at least fixes the failing
> > > > > > selftest reported by Alexei for ORC.
> > > > > >
> > > > > > How can I recreate your issue?
> > > > >
> > > > > Hmm, I used bcc's example to attach bpf to trace point, and with that
> > > > > fix stack trace is still invalid.
> > > > >
> > > > > CMD I used with bcc:
> > > > > python3 ./tools/stackcount.py t:sched:sched_fork
> > > >
> > > > I've had problems in the past getting bcc to build, so I was hoping it
> > > > was reproducible with a standalone selftest.
> > > >
> > > > > And I just had another try applying your patch, self test is also failing.
> > > >
> > > > Is it the same selftest reported by Alexei?
> > > >
> > > > test_stacktrace_map:FAIL:compare_map_keys stackid_hmap vs. stackmap err -1 errno 2
> > > >
> > > > > I'm applying on my local master branch, a few days older than
> > > > > upstream, I can update and try again, am I missing anything?
> > > >
> > > > The above patch had some issues, so with some configs you might see an
> > > > objtool warning for ___bpf_prog_run(), in which case the patch doesn't
> > > > fix the test_stacktrace_map selftest.
> > > >
> > > > Here's the latest version which should fix it in all cases (based on
> > > > tip/master):
> > > >
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/commit/?h=bpf-orc-fix
> > >
> > > Hmm, I still get the failure:
> > > test_stacktrace_map:FAIL:compare_map_keys stackid_hmap vs. stackmap
> > > err -1 errno 2
> > >
> > > And I didn't see how this will fix the issue. As long as ORC need to
> > > unwind through the JITed code it will fail. And that will happen
> > > before reaching ___bpf_prog_run.
> >
> > Ok, I was able to recreate by doing
> >
> > echo 1 > /proc/sys/net/core/bpf_jit_enable
> >
> > first. I'm guessing you have CONFIG_BPF_JIT_ALWAYS_ON.
> >
>
> Yes, with JIT off it will be fixed. I can confirm that.

Here's a tentative BPF fix for the JIT frame pointer issue. It was a
bit harder than I expected. Encoding r12 as a base register requires a
SIB byte, so I had to add support for encoding that. I also simplified
the prologue to resemble a GCC prologue, which decreases the prologue
size quite a bit.

Next week I can work on the corresponding ORC change. Then I can clean
all the patches up and submit them properly.

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index afabf597c855..c9b4503558c9 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -104,9 +104,8 @@ static int bpf_size_to_x86_bytes(int bpf_size)
/*
* The following table maps BPF registers to x86-64 registers.
*
- * x86-64 register R12 is unused, since if used as base address
- * register in load/store instructions, it always needs an
- * extra byte of encoding and is callee saved.
+ * RBP isn't used; it needs to be preserved to allow the unwinder to move
+ * through generated code stacks.
*
* Also x86-64 register R9 is unused. x86-64 register R10 is
* used for blinding (if enabled).
@@ -122,7 +121,7 @@ static const int reg2hex[] = {
[BPF_REG_7] = 5, /* R13 callee saved */
[BPF_REG_8] = 6, /* R14 callee saved */
[BPF_REG_9] = 7, /* R15 callee saved */
- [BPF_REG_FP] = 5, /* RBP readonly */
+ [BPF_REG_FP] = 4, /* R12 readonly */
[BPF_REG_AX] = 2, /* R10 temp register */
[AUX_REG] = 3, /* R11 temp register */
};
@@ -139,6 +138,7 @@ static bool is_ereg(u32 reg)
BIT(BPF_REG_7) |
BIT(BPF_REG_8) |
BIT(BPF_REG_9) |
+ BIT(BPF_REG_FP) |
BIT(BPF_REG_AX));
}

@@ -147,6 +147,11 @@ static bool is_axreg(u32 reg)
return reg == BPF_REG_0;
}

+static bool is_sib_reg(u32 reg)
+{
+ return reg == BPF_REG_FP;
+}
+
/* Add modifiers if 'reg' maps to x86-64 registers R8..R15 */
static u8 add_1mod(u8 byte, u32 reg)
{
@@ -190,15 +195,13 @@ struct jit_context {
#define BPF_MAX_INSN_SIZE 128
#define BPF_INSN_SAFETY 64

-#define AUX_STACK_SPACE 40 /* Space for RBX, R13, R14, R15, tailcnt */
-
-#define PROLOGUE_SIZE 37
+#define PROLOGUE_SIZE 25

/*
* Emit x86-64 prologue code for BPF program and check its size.
* bpf_tail_call helper will skip it while jumping into another program
*/
-static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf)
+static void emit_prologue(u8 **pprog, u32 stack_depth)
{
u8 *prog = *pprog;
int cnt = 0;
@@ -206,40 +209,67 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf)
/* push rbp */
EMIT1(0x55);

- /* mov rbp,rsp */
+ /* mov rbp, rsp */
EMIT3(0x48, 0x89, 0xE5);

- /* sub rsp, rounded_stack_depth + AUX_STACK_SPACE */
- EMIT3_off32(0x48, 0x81, 0xEC,
- round_up(stack_depth, 8) + AUX_STACK_SPACE);
+ /* push r15 */
+ EMIT2(0x41, 0x57);
+ /* push r14 */
+ EMIT2(0x41, 0x56);
+ /* push r13 */
+ EMIT2(0x41, 0x55);
+ /* push r12 */
+ EMIT2(0x41, 0x54);
+ /* push rbx */
+ EMIT1(0x53);

- /* sub rbp, AUX_STACK_SPACE */
- EMIT4(0x48, 0x83, 0xED, AUX_STACK_SPACE);
+ /*
+ * Push the tail call counter (tail_call_cnt) for eBPF tail calls.
+ * Initialized to zero.
+ *
+ * push $0
+ */
+ EMIT2(0x6a, 0x00);

- /* mov qword ptr [rbp+0],rbx */
- EMIT4(0x48, 0x89, 0x5D, 0);
- /* mov qword ptr [rbp+8],r13 */
- EMIT4(0x4C, 0x89, 0x6D, 8);
- /* mov qword ptr [rbp+16],r14 */
- EMIT4(0x4C, 0x89, 0x75, 16);
- /* mov qword ptr [rbp+24],r15 */
- EMIT4(0x4C, 0x89, 0x7D, 24);
+ /*
+ * R12 is used for the BPF program's FP register. It points to the end
+ * of the program's stack area.
+ *
+ * mov r12, rsp
+ */
+ EMIT3(0x49, 0x89, 0xE4);

- if (!ebpf_from_cbpf) {
- /*
- * Clear the tail call counter (tail_call_cnt): for eBPF tail
- * calls we need to reset the counter to 0. It's done in two
- * instructions, resetting RAX register to 0, and moving it
- * to the counter location.
- */
+ /* sub rsp, rounded_stack_depth */
+ EMIT3_off32(0x48, 0x81, 0xEC, round_up(stack_depth, 8));

- /* xor eax, eax */
- EMIT2(0x31, 0xc0);
- /* mov qword ptr [rbp+32], rax */
- EMIT4(0x48, 0x89, 0x45, 32);
+ BUILD_BUG_ON(cnt != PROLOGUE_SIZE);

- BUILD_BUG_ON(cnt != PROLOGUE_SIZE);
- }
+ *pprog = prog;
+}
+
+static void emit_epilogue(u8 **pprog)
+{
+ u8 *prog = *pprog;
+ int cnt = 0;
+
+ /* lea rsp, [rbp-0x28] */
+ EMIT4(0x48, 0x8D, 0x65, 0xD8);
+
+ /* pop rbx */
+ EMIT1(0x5B);
+ /* pop r12 */
+ EMIT2(0x41, 0x5C);
+ /* pop r13 */
+ EMIT2(0x41, 0x5D);
+ /* pop r14 */
+ EMIT2(0x41, 0x5E);
+ /* pop r15 */
+ EMIT2(0x41, 0x5F);
+ /* pop rbp */
+ EMIT1(0x5D);
+
+ /* ret */
+ EMIT1(0xC3);

*pprog = prog;
}
@@ -277,7 +307,7 @@ static void emit_bpf_tail_call(u8 **pprog)
EMIT2(0x89, 0xD2); /* mov edx, edx */
EMIT3(0x39, 0x56, /* cmp dword ptr [rsi + 16], edx */
offsetof(struct bpf_array, map.max_entries));
-#define OFFSET1 (41 + RETPOLINE_RAX_BPF_JIT_SIZE) /* Number of bytes to jump */
+#define OFFSET1 (35 + RETPOLINE_RAX_BPF_JIT_SIZE) /* Number of bytes to jump */
EMIT2(X86_JBE, OFFSET1); /* jbe out */
label1 = cnt;

@@ -285,13 +315,13 @@ static void emit_bpf_tail_call(u8 **pprog)
* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
* goto out;
*/
- EMIT2_off32(0x8B, 0x85, 36); /* mov eax, dword ptr [rbp + 36] */
+ EMIT3(0x8B, 0x45, 0xD4); /* mov eax, dword ptr [rbp - 44] */
EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT); /* cmp eax, MAX_TAIL_CALL_CNT */
-#define OFFSET2 (30 + RETPOLINE_RAX_BPF_JIT_SIZE)
+#define OFFSET2 (27 + RETPOLINE_RAX_BPF_JIT_SIZE)
EMIT2(X86_JA, OFFSET2); /* ja out */
label2 = cnt;
EMIT3(0x83, 0xC0, 0x01); /* add eax, 1 */
- EMIT2_off32(0x89, 0x85, 36); /* mov dword ptr [rbp + 36], eax */
+ EMIT3(0x89, 0x45, 0xD4); /* mov dword ptr [rbp - 44], eax */

/* prog = array->ptrs[index]; */
EMIT4_off32(0x48, 0x8B, 0x84, 0xD6, /* mov rax, [rsi + rdx * 8 + offsetof(...)] */
@@ -419,8 +449,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
int proglen = 0;
u8 *prog = temp;

- emit_prologue(&prog, bpf_prog->aux->stack_depth,
- bpf_prog_was_classic(bpf_prog));
+ emit_prologue(&prog, bpf_prog->aux->stack_depth);

for (i = 0; i < insn_cnt; i++, insn++) {
const s32 imm32 = insn->imm;
@@ -767,10 +796,19 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
case BPF_ST | BPF_MEM | BPF_DW:
EMIT2(add_1mod(0x48, dst_reg), 0xC7);

-st: if (is_imm8(insn->off))
- EMIT2(add_1reg(0x40, dst_reg), insn->off);
+st:
+ if (is_imm8(insn->off))
+ EMIT1(add_1reg(0x40, dst_reg));
else
- EMIT1_off32(add_1reg(0x80, dst_reg), insn->off);
+ EMIT1(add_1reg(0x80, dst_reg));
+
+ if (is_sib_reg(dst_reg))
+ EMIT1(add_1reg(0x20, dst_reg));
+
+ if (is_imm8(insn->off))
+ EMIT1(insn->off);
+ else
+ EMIT(insn->off, 4);

EMIT(imm32, bpf_size_to_x86_bytes(BPF_SIZE(insn->code)));
break;
@@ -799,11 +837,19 @@ st: if (is_imm8(insn->off))
goto stx;
case BPF_STX | BPF_MEM | BPF_DW:
EMIT2(add_2mod(0x48, dst_reg, src_reg), 0x89);
-stx: if (is_imm8(insn->off))
- EMIT2(add_2reg(0x40, dst_reg, src_reg), insn->off);
+stx:
+ if (is_imm8(insn->off))
+ EMIT1(add_2reg(0x40, dst_reg, src_reg));
+ else
+ EMIT1(add_2reg(0x80, dst_reg, src_reg));
+
+ if (is_sib_reg(dst_reg))
+ EMIT1(add_1reg(0x20, dst_reg));
+
+ if (is_imm8(insn->off))
+ EMIT1(insn->off);
else
- EMIT1_off32(add_2reg(0x80, dst_reg, src_reg),
- insn->off);
+ EMIT(insn->off, 4);
break;

/* LDX: dst_reg = *(u8*)(src_reg + off) */
@@ -825,16 +871,24 @@ stx: if (is_imm8(insn->off))
case BPF_LDX | BPF_MEM | BPF_DW:
/* Emit 'mov rax, qword ptr [rax+0x14]' */
EMIT2(add_2mod(0x48, src_reg, dst_reg), 0x8B);
-ldx: /*
+ldx:
+ /*
* If insn->off == 0 we can save one extra byte, but
* special case of x86 R13 which always needs an offset
* is not worth the hassle
*/
if (is_imm8(insn->off))
- EMIT2(add_2reg(0x40, src_reg, dst_reg), insn->off);
+ EMIT1(add_2reg(0x40, src_reg, dst_reg));
else
- EMIT1_off32(add_2reg(0x80, src_reg, dst_reg),
- insn->off);
+ EMIT1(add_2reg(0x80, src_reg, dst_reg));
+
+ if (is_sib_reg(src_reg))
+ EMIT1(add_1reg(0x20, src_reg));
+
+ if (is_imm8(insn->off))
+ EMIT1(insn->off);
+ else
+ EMIT(insn->off, 4);
break;

/* STX XADD: lock *(u32*)(dst_reg + off) += src_reg */
@@ -847,11 +901,19 @@ stx: if (is_imm8(insn->off))
goto xadd;
case BPF_STX | BPF_XADD | BPF_DW:
EMIT3(0xF0, add_2mod(0x48, dst_reg, src_reg), 0x01);
-xadd: if (is_imm8(insn->off))
- EMIT2(add_2reg(0x40, dst_reg, src_reg), insn->off);
+xadd:
+ if (is_imm8(insn->off))
+ EMIT1(add_2reg(0x40, dst_reg, src_reg));
else
- EMIT1_off32(add_2reg(0x80, dst_reg, src_reg),
- insn->off);
+ EMIT1(add_2reg(0x80, dst_reg, src_reg));
+
+ if (is_sib_reg(dst_reg))
+ EMIT1(add_1reg(0x20, dst_reg));
+
+ if (is_imm8(insn->off))
+ EMIT1(insn->off);
+ else
+ EMIT(insn->off, 4);
break;

/* call */
@@ -1040,19 +1102,8 @@ xadd: if (is_imm8(insn->off))
seen_exit = true;
/* Update cleanup_addr */
ctx->cleanup_addr = proglen;
- /* mov rbx, qword ptr [rbp+0] */
- EMIT4(0x48, 0x8B, 0x5D, 0);
- /* mov r13, qword ptr [rbp+8] */
- EMIT4(0x4C, 0x8B, 0x6D, 8);
- /* mov r14, qword ptr [rbp+16] */
- EMIT4(0x4C, 0x8B, 0x75, 16);
- /* mov r15, qword ptr [rbp+24] */
- EMIT4(0x4C, 0x8B, 0x7D, 24);
-
- /* add rbp, AUX_STACK_SPACE */
- EMIT4(0x48, 0x83, 0xC5, AUX_STACK_SPACE);
- EMIT1(0xC9); /* leave */
- EMIT1(0xC3); /* ret */
+
+ emit_epilogue(&prog);
break;

default:
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