Re: Getting empty callchain from perf_callchain_kernel()
From: Kairui Song
Date: Mon May 27 2019 - 08:01:06 EST
On Sat, May 25, 2019 at 7:23 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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:
Thanks! This looks good to me and passed the self test and bcc test
(with frame pointer unwinder, and JIT enabled):
With bcc's tools/stackcount.py I got the valid stack trace, and the
self test says:
test_stacktrace_map:PASS:compare_map_keys stackid_hmap vs. stackmap 0 nsec
--
Best Regards,
Kairui Song