As BPF applications grow in size and complexity and are separated into
multiple .bpf.c files that are statically linked together, it becomes
harder and harder to match verifier's BPF assembly level output to
original C code. While often annotated C source code is unique enough to
be able to identify the file it belongs to, quite often this is actually
problematic as parts of source code can be quite generic.
Long story short, it is very useful to see source code file name and
line number information along with the original C code. Verifier already
knows this information, we just need to output it.
This patch set is an initial proposal on how this can be done. No new
flags are added and file:line information is appended at the end of
C code:
; <original C code> (<filename>.bpf.c:<line>)
If file name has directory names in it, they are stripped away. This
should be fine in practice as file names tend to be pretty unique with
C code anyways, and keeping log size smaller is always good.
In practice this might look something like below, where some code is
coming from application files, while others are from libbpf's usdt.bpf.h
header file:
; if (STROBEMETA_READ( (strobemeta_probe.bpf.c:534)
5592: (79) r1 = *(u64 *)(r10 -56) ; R1_w=mem_or_null(id=1589,sz=7680) R10=fp0 fp-56_w=mem_or_null(id=1589,sz=7680)
5593: (7b) *(u64 *)(r10 -56) = r1 ; R1_w=mem_or_null(id=1589,sz=7680) R10=fp0 fp-56_w=mem_or_null(id=1589,sz=7680)
5594: (79) r3 = *(u64 *)(r10 -8) ; R3_w=scalar() R10=fp0 fp-8=mmmmmmmm
...
170: (71) r1 = *(u8 *)(r8 +15) ; frame1: R1_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=255,var_off=(0x0; 0xff)) R8_w=map_value(map=__bpf_usdt_spec,ks=4,vs=208)
171: (67) r1 <<= 56 ; frame1: R1_w=scalar(smax=0x7f00000000000000,umax=0xff00000000000000,smin32=0,smax32=umax32=0,var_off=(0x0; 0xff00000000000000))
172: (c7) r1 s>>= 56 ; frame1: R1_w=scalar(smin=smin32=-128,smax=smax32=127)
; val <<= arg_spec->arg_bitshift; (usdt.bpf.h:183)
173: (67) r1 <<= 32 ; frame1: R1_w=scalar(smax=0x7f00000000,umax=0xffffffff00000000,smin32=0,smax32=umax32=0,var_off=(0x0; 0xffffffff00000000))
174: (77) r1 >>= 32 ; frame1: R1_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
175: (79) r2 = *(u64 *)(r10 -8) ; frame1: R2_w=scalar() R10=fp0 fp-8=mmmmmmmm
176: (6f) r2 <<= r1 ; frame1: R1_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff)) R2_w=scalar()
177: (7b) *(u64 *)(r10 -8) = r2 ; frame1: R2_w=scalar(id=61) R10=fp0 fp-8_w=scalar(id=61)
; if (arg_spec->arg_signed) (usdt.bpf.h:184)
178: (bf) r3 = r2 ; frame1: R2_w=scalar(id=61) R3_w=scalar(id=61)
179: (7f) r3 >>= r1 ; frame1: R1_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff)) R3_w=scalar()
; if (arg_spec->arg_signed) (usdt.bpf.h:184)
180: (71) r4 = *(u8 *)(r8 +14)
181: safe
I've played with few different formats and none stood out as
particularly better than other. Suggestions and votes are appreciated:
a) ; if (arg_spec->arg_signed) (usdt.bpf.h:184)
b) ; if (arg_spec->arg_signed) [usdt.bpf.h:184]
c) ; [usdt.bpf.h:184] if (arg_spec->arg_signed)
d) ; (usdt.bpf.h:184) if (arg_spec->arg_signed)
Above output shows variant a), which is quite non-distracting in
practice.
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
kernel/bpf/log.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/kernel/bpf/log.c b/kernel/bpf/log.c
index 594a234f122b..4b49d0eb5cd1 100644
--- a/kernel/bpf/log.c
+++ b/kernel/bpf/log.c
@@ -9,6 +9,7 @@
#include <linux/bpf.h>
#include <linux/bpf_verifier.h>
#include <linux/math64.h>
+#include <linux/string.h>
#define verbose(env, fmt, args...) bpf_verifier_log_write(env, fmt, ##args)
@@ -362,6 +363,8 @@ __printf(3, 4) void verbose_linfo(struct bpf_verifier_env *env,
const char *prefix_fmt, ...)
{
const struct bpf_line_info *linfo;
+ const struct btf *btf;
+ const char *s, *fname;
if (!bpf_verifier_log_needed(&env->log))
return;
@@ -378,9 +381,15 @@ __printf(3, 4) void verbose_linfo(struct bpf_verifier_env *env,
va_end(args);
}
- verbose(env, "%s\n",
- ltrim(btf_name_by_offset(env->prog->aux->btf,
- linfo->line_off)));
+ btf = env->prog->aux->btf;
+ s = ltrim(btf_name_by_offset(btf, linfo->line_off));
+ verbose(env, "%s", s); /* source code line */
+
+ s = btf_name_by_offset(btf, linfo->file_name_off);
+ /* leave only file name */
+ fname = strrchr(s, '/');
+ fname = fname ? fname + 1 : s;
+ verbose(env, " (%s:%u)\n", fname, BPF_LINE_INFO_LINE_NUM(linfo->line_col));
env->prev_linfo = linfo;
}
--
2.39.3
