2022-10-20 10:49 UTC-0700 ~ Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx>
> On Thu, Oct 20, 2022 at 5:37 AM Quentin Monnet <quentin@xxxxxxxxxxxxx> wrote:
>>
>> +
>> +/* This callback to set the ref_type is necessary to have the LLVM disassembler
>> + * print PC-relative addresses instead of byte offsets for branch instruction
>> + * targets.
>> + */
>> +static const char *
>> +symbol_lookup_callback(__maybe_unused void *disasm_info,
>> + __maybe_unused uint64_t ref_value,
>> + uint64_t *ref_type, __maybe_unused uint64_t ref_PC,
>> + __maybe_unused const char **ref_name)
>> +{
>> + *ref_type = LLVMDisassembler_ReferenceType_InOut_None;
>> + return NULL;
>> +}
>
> Could you give an example before/after for asm
> that contains 'call foo' instructions?
> I'm not sure that above InOut_None will not break
> symbolization.
Hi Alexei, I ran a quick test and it doesn't seem we lose any
information. Building from:
#include <linux/bpf.h>
#include "bpf_helper_defs.h"
#define SEC(name) __attribute__((section(name), used))
static __attribute__((noinline))
int bar(int b) {
return bpf_get_prandom_u32() > b;
}
SEC("xdp")
int foo(struct xdp_md *ctx) {
void *data = (void *)(long)ctx->data;
void *data_end = (void *)(long)ctx->data_end;
return bar(data_end - data);
}
Here is the output from the existing version (using libbfd):
# bpftool version
bpftool v7.1.0
using libbpf v1.1
features: libbfd, libbpf_strict, skeletons
# bpftool prog dump jited name foo
int foo(struct xdp_md * ctx):
bpf_prog_65e359e7b0251046_foo:
; void *data = (void *)(long)ctx->data;
0: nopl 0x0(%rax,%rax,1)
5: xchg %ax,%ax
7: push %rbp
8: mov %rsp,%rbp
b: mov 0x0(%rdi),%rsi
; void *data_end = (void *)(long)ctx->data_end;
f: mov 0x8(%rdi),%rdi
; return bar(data_end - data);
13: sub %esi,%edi
; return bar(data_end - data);
15: call 0x0000000000000038
; return bar(data_end - data);
1a: leave
1b: ret
int bar(int b):
bpf_prog_9b001d67a67f01cc_bar:
; int bar(int b) {
0: nopl 0x0(%rax,%rax,1)
5: xchg %ax,%ax
7: push %rbp
8: mov %rsp,%rbp
b: push %rbx
c: mov %edi,%ebx
; return bpf_get_prandom_u32() > b;
e: call 0xffffffffcab00454
13: mov %eax,%edi
15: mov $0x1,%eax
; return bpf_get_prandom_u32() > b;
1a: cmp %ebx,%edi
1c: ja 0x0000000000000020
1e: xor %eax,%eax
; return bpf_get_prandom_u32() > b;
20: pop %rbx
21: leave
22: ret
Did you expect "bar" to appear on insn '15:'? I don't think we get this
from bpftool at the moment? Or did I misunderstand your question?
The output from LLVM's disassembler comes below:
# ./bpftool version
bpftool v7.1.0
using libbpf v1.1
features: llvm, libbpf_strict, skeletons
# ./bpftool prog dump jited name foo
int foo(struct xdp_md * ctx):
bpf_prog_65e359e7b0251046_foo:
; void *data = (void *)(long)ctx->data;
0: nopl (%rax,%rax)
5: nop
7: pushq %rbp
8: movq %rsp, %rbp
b: movq (%rdi), %rsi
; void *data_end = (void *)(long)ctx->data_end;
f: movq 8(%rdi), %rdi
; return bar(data_end - data);
13: subl %esi, %edi
; return bar(data_end - data);
15: callq 0x38
; return bar(data_end - data);
1a: leave
1b: retq
int bar(int b):
bpf_prog_9b001d67a67f01cc_bar:
; int bar(int b) {
0: nopl (%rax,%rax)
5: nop
7: pushq %rbp
8: movq %rsp, %rbp
b: pushq %rbx
c: movl %edi, %ebx
; return bpf_get_prandom_u32() > b;
e: callq 0xffffffffcab00454
13: movl %eax, %edi
15: movl $1, %eax
; return bpf_get_prandom_u32() > b;
1a: cmpl %ebx, %edi
1c: ja 0x20
1e: xorl %eax, %eax
; return bpf_get_prandom_u32() > b;
20: popq %rbx
21: leave
22: retq
LLVM, but _without_ the LLVMDisassembler_ReferenceType_InOut_None:
int foo(struct xdp_md * ctx):
bpf_prog_65e359e7b0251046_foo:
; void *data = (void *)(long)ctx->data;
0: nopl (%rax,%rax)
5: nop
7: pushq %rbp
8: movq %rsp, %rbp
b: movq (%rdi), %rsi
; void *data_end = (void *)(long)ctx->data_end;
f: movq 8(%rdi), %rdi
; return bar(data_end - data);
13: subl %esi, %edi
; return bar(data_end - data);
15: callq 30
; return bar(data_end - data);
1a: leave
1b: retq
int bar(int b):
bpf_prog_9b001d67a67f01cc_bar:
; int bar(int b) {
0: nopl (%rax,%rax)
5: nop
7: pushq %rbp
8: movq %rsp, %rbp
b: pushq %rbx
c: movl %edi, %ebx
; return bpf_get_prandom_u32() > b;
e: callq -894434239
13: movl %eax, %edi
15: movl $1, %eax
; return bpf_get_prandom_u32() > b;
1a: cmpl %ebx, %edi
1c: ja 2
1e: xorl %eax, %eax
; return bpf_get_prandom_u32() > b;
20: popq %rbx
21: leave
22: retq
