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Date: Fri, 14 Jun 2019 12:07:20 -0500
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: David Laight <David.Laight@...LAB.COM>
Cc: 'Alexei Starovoitov' <alexei.starovoitov@...il.com>,
"x86@...nel.org" <x86@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"bpf@...r.kernel.org" <bpf@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Song Liu <songliubraving@...com>,
Kairui Song <kasong@...hat.com>
Subject: Re: [PATCH 6/9] x86/bpf: Fix JIT frame pointer usage
On Fri, Jun 14, 2019 at 01:58:21PM +0000, David Laight wrote:
> From: Josh Poimboeuf
> > Sent: 14 June 2019 14:44
> >
> > On Fri, Jun 14, 2019 at 10:50:23AM +0000, David Laight wrote:
> > > On Thu, Jun 13, 2019 at 08:21:03AM -0500, Josh Poimboeuf wrote:
> > > > The BPF JIT code clobbers RBP. This breaks frame pointer convention and
> > > > thus prevents the FP unwinder from unwinding through JIT generated code.
> > > >
> > > > RBP is currently used as the BPF stack frame pointer register. The
> > > > actual register used is opaque to the user, as long as it's a
> > > > callee-saved register. Change it to use R12 instead.
> > >
> > > Could you maintain the system %rbp chain through the BPF stack?
> >
> > Do you mean to save RBP again before changing it again, so that we
> > create another stack frame inside the BPF stack? That might work.
>
> The unwinder will (IIRC) expect *%rbp to be the previous %rbp value.
> If you maintain that it will probably all work.
>
> > > It might even be possible to put something relevant in the %rip
> > > location.
> >
> > I'm not sure what you mean here.
>
> The return address is (again IIRC) %rbp[-8] so the unwinder will
> expect that address to be a symbol.
Ah, gotcha. We don't necessarily need the real rip on the stack as the
unwinder can handle bad text addresses ok. Though the real one would be
better.
> I do remember a stack trace printer for x86 this didn't need
> any annotation of the object code and didn't need frame pointers.
> The only downside was that it had to 'guess' (ie scan the stack)
> to get out of functions that couldn't return.
> Basically it followed the control flow forwards tracking the
> values of %sp and %bp until it found a return instuction.
> All it has to do is detect loops and retry from the other
> target of conditional branches.
That actually sounds kind of cool, though I don't think we need that for
the kernel.
Anyway here's a patch with your suggestion. I think it's the best idea
so far because it doesn't require the use of R12, nor does it require
abstracting BPF_REG_FP with an offset. And the diffstat is pretty
small and self-contained.
It seems to work, though I didn't put a real RIP on the stack yet. This
is based on top of the "x86/bpf: Simplify prologue generation" patch.
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 485692d4b163..fa1fe65c4cb4 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -186,7 +186,7 @@ struct jit_context {
#define BPF_MAX_INSN_SIZE 128
#define BPF_INSN_SAFETY 64
-#define PROLOGUE_SIZE 20
+#define PROLOGUE_SIZE 24
/*
* Emit x86-64 prologue code for BPF program and check its size.
@@ -197,14 +197,17 @@ static void emit_prologue(u8 **pprog, u32 stack_depth)
u8 *prog = *pprog;
int cnt = 0;
+ /* push rbp */
+ EMIT1(0x55);
+ /* mov rbp, rsp */
+ EMIT3(0x48, 0x89, 0xE5);
+
/* push r15 */
EMIT2(0x41, 0x57);
/* push r14 */
EMIT2(0x41, 0x56);
/* push r13 */
EMIT2(0x41, 0x55);
- /* push rbp */
- EMIT1(0x55);
/* push rbx */
EMIT1(0x53);
@@ -218,10 +221,13 @@ static void emit_prologue(u8 **pprog, u32 stack_depth)
/*
* RBP is used for the BPF program's FP register. It points to the end
- * of the program's stack area.
- *
- * mov rbp, rsp
+ * of the program's stack area. Create another stack frame so the
+ * unwinder can unwind through the generated code. The tail_call_cnt
+ * value doubles as an (invalid) RIP address.
*/
+ /* push rbp */
+ EMIT1(0x55);
+ /* mov rbp, rsp */
EMIT3(0x48, 0x89, 0xE5);
/* sub rsp, rounded_stack_depth */
@@ -237,19 +243,21 @@ static void emit_epilogue(u8 **pprog)
u8 *prog = *pprog;
int cnt = 0;
- /* lea rsp, [rbp+0x8] */
- EMIT4(0x48, 0x8D, 0x65, 0x08);
+ /* leave (restore rsp and rbp) */
+ EMIT1(0xC9);
+ /* pop rbx (skip over tail_call_cnt) */
+ EMIT1(0x5B);
/* pop rbx */
EMIT1(0x5B);
- /* pop rbp */
- EMIT1(0x5D);
/* pop r13 */
EMIT2(0x41, 0x5D);
/* pop r14 */
EMIT2(0x41, 0x5E);
/* pop r15 */
EMIT2(0x41, 0x5F);
+ /* pop rbp */
+ EMIT1(0x5D);
/* ret */
EMIT1(0xC3);
@@ -298,13 +306,13 @@ static void emit_bpf_tail_call(u8 **pprog)
* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
* goto out;
*/
- EMIT3(0x8B, 0x45, 0x04); /* mov eax, dword ptr [rbp + 4] */
+ EMIT3(0x8B, 0x45, 0x0C); /* mov eax, dword ptr [rbp + 12] */
EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT); /* cmp eax, MAX_TAIL_CALL_CNT */
#define OFFSET2 (27 + RETPOLINE_RAX_BPF_JIT_SIZE)
EMIT2(X86_JA, OFFSET2); /* ja out */
label2 = cnt;
EMIT3(0x83, 0xC0, 0x01); /* add eax, 1 */
- EMIT3(0x89, 0x45, 0x04); /* mov dword ptr [rbp + 4], eax */
+ EMIT3(0x89, 0x45, 0x0C); /* mov dword ptr [rbp + 12], eax */
/* prog = array->ptrs[index]; */
EMIT4_off32(0x48, 0x8B, 0x84, 0xD6, /* mov rax, [rsi + rdx * 8 + offsetof(...)] */
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