Re: [PATCH 23/33] x86/asm/bpf: Create stack frames in bpf_jit.S
From: Alexei Starovoitov
Date: Thu Jan 21 2016 - 21:44:41 EST
On Thu, Jan 21, 2016 at 04:49:27PM -0600, Josh Poimboeuf wrote:
> bpf_jit.S has several callable non-leaf functions which don't honor
> CONFIG_FRAME_POINTER, which can result in bad stack traces.
> Create a stack frame before the call instructions when
> CONFIG_FRAME_POINTER is enabled.
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx
> arch/x86/net/bpf_jit.S | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
> diff --git a/arch/x86/net/bpf_jit.S b/arch/x86/net/bpf_jit.S
> index eb4a3bd..f2a7faf 100644
> --- a/arch/x86/net/bpf_jit.S
> +++ b/arch/x86/net/bpf_jit.S
> @@ -8,6 +8,7 @@
> * of the License.
> #include <linux/linkage.h>
> +#include <asm/frame.h>
> * Calling convention :
> @@ -65,16 +66,18 @@ FUNC(sk_load_byte_positive_offset)
> /* rsi contains offset and can be scratched */
> #define bpf_slow_path_common(LEN) \
> + lea -MAX_BPF_STACK + 32(%rbp), %rdx;\
> + FRAME_BEGIN; \
> mov %rbx, %rdi; /* arg1 == skb */ \
> push %r9; \
> push SKBDATA; \
> /* rsi already has offset */ \
> mov $LEN,%ecx; /* len */ \
> - lea - MAX_BPF_STACK + 32(%rbp),%rdx; \
> call skb_copy_bits; \
> test %eax,%eax; \
> pop SKBDATA; \
> - pop %r9;
> + pop %r9; \
> + FRAME_END
I'm not sure what above is doing.
There is already 'push rbp; mov rbp,rsp' at the beginning of generated
code and with above the stack trace will show two function at the same ip?
since there were no calls between them?
I think the stack walker will get even more confused?
Also the JIT of bpf_call insn will emit variable number of push/pop
around the call and I definitely don't want to add extra push rbp
there, since it's the critical path and callee will do its own
Also there are push/pops emitted around div/mod
and there is indirect goto emitted as well for bpf_tail_call
that jumps into different function body without touching
Also none of the JITed function are dwarf annotated.
I could be missing something. I think either this patch
is not need or you need to teach the tool to ignore
all JITed stuff. I don't think it's practical to annotate
everything. Different JITs do their own magic.
s390 JIT is even more fancy.