diff options
| author | Gianluca Borello <g.borello@gmail.com> | 2017-11-22 18:32:56 +0000 |
|---|---|---|
| committer | Daniel Borkmann <daniel@iogearbox.net> | 2017-11-22 21:40:54 +0100 |
| commit | a60dd35d2e39209fa7645945e1192bf9769872c6 (patch) | |
| tree | 1606e891bab8ee8bf102c3f0a3b4319d81a0ef2d /kernel/trace/bpf_trace.c | |
| parent | 5c4e1201740ceae9bd6f622851a9bf7c66debe3a (diff) | |
| download | lwn-a60dd35d2e39209fa7645945e1192bf9769872c6.tar.gz lwn-a60dd35d2e39209fa7645945e1192bf9769872c6.zip | |
bpf: change bpf_perf_event_output arg5 type to ARG_CONST_SIZE_OR_ZERO
Commit 9fd29c08e520 ("bpf: improve verifier ARG_CONST_SIZE_OR_ZERO
semantics") relaxed the treatment of ARG_CONST_SIZE_OR_ZERO due to the way
the compiler generates optimized BPF code when checking boundaries of an
argument from C code. A typical example of this optimized code can be
generated using the bpf_perf_event_output helper when operating on variable
memory:
/* len is a generic scalar */
if (len > 0 && len <= 0x7fff)
bpf_perf_event_output(ctx, &perf_map, 0, buf, len);
110: (79) r5 = *(u64 *)(r10 -40)
111: (bf) r1 = r5
112: (07) r1 += -1
113: (25) if r1 > 0x7ffe goto pc+6
114: (bf) r1 = r6
115: (18) r2 = 0xffff94e5f166c200
117: (b7) r3 = 0
118: (bf) r4 = r7
119: (85) call bpf_perf_event_output#25
R5 min value is negative, either use unsigned or 'var &= const'
With this code, the verifier loses track of the variable.
Replacing arg5 with ARG_CONST_SIZE_OR_ZERO is thus desirable since it
avoids this quite common case which leads to usability issues, and the
compiler generates code that the verifier can more easily test:
if (len <= 0x7fff)
bpf_perf_event_output(ctx, &perf_map, 0, buf, len);
or
bpf_perf_event_output(ctx, &perf_map, 0, buf, len & 0x7fff);
No changes to the bpf_perf_event_output helper are necessary since it can
handle a case where size is 0, and an empty frame is pushed.
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Gianluca Borello <g.borello@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Diffstat (limited to 'kernel/trace/bpf_trace.c')
| -rw-r--r-- | kernel/trace/bpf_trace.c | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index ed8601a1a861..27d1f4ffa3de 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -403,7 +403,7 @@ static const struct bpf_func_proto bpf_perf_event_output_proto = { .arg2_type = ARG_CONST_MAP_PTR, .arg3_type = ARG_ANYTHING, .arg4_type = ARG_PTR_TO_MEM, - .arg5_type = ARG_CONST_SIZE, + .arg5_type = ARG_CONST_SIZE_OR_ZERO, }; static DEFINE_PER_CPU(struct pt_regs, bpf_pt_regs); @@ -605,7 +605,7 @@ static const struct bpf_func_proto bpf_perf_event_output_proto_tp = { .arg2_type = ARG_CONST_MAP_PTR, .arg3_type = ARG_ANYTHING, .arg4_type = ARG_PTR_TO_MEM, - .arg5_type = ARG_CONST_SIZE, + .arg5_type = ARG_CONST_SIZE_OR_ZERO, }; BPF_CALL_3(bpf_get_stackid_tp, void *, tp_buff, struct bpf_map *, map, |