On Wed, 2023-01-25 at 16:26 -0800, Andrii Nakryiko wrote:
> On Wed, Jan 25, 2023 at 1:39 PM Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> wrote:
> >
> > Loading programs that use bpf_usdt_arg() on s390x fails with:
> >
> > ; switch (arg_spec->arg_type) {
> > 139: (61) r1 = *(u32 *)(r2 +8)
> > R2 unbounded memory access, make sure to bounds check any such
> > access
>
> can you show a bit longer log? we shouldn't just use
> bpf_probe_read_kernel for this. I suspect strategically placed
> barrier_var() calls will solve this. This is usually an issue with
> compiler reordering operations and doing actual check after it
> already
> speculatively adjusted pointer (which is technically safe and ok if
> we
> never deref that pointer, but verifier doesn't recognize such
> pattern)
The full log is here:
https://gist.github.com/iii-i/b6149ee99b37078ec920ab1d3bb45134
The relevant part seems to be:
; if (arg_num >= BPF_USDT_MAX_ARG_CNT || arg_num >= spec->arg_cnt)
128: (79) r1 = *(u64 *)(r10 -24) ; frame1:
R1_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
129: (25) if r1 > 0xb goto pc+83 ; frame1:
R1_w=scalar(umax=11,var_off=(0x0; 0xf))
; if (arg_num >= BPF_USDT_MAX_ARG_CNT || arg_num >= spec->arg_cnt)
130: (69) r1 = *(u16 *)(r8 +200) ; frame1:
R1_w=scalar(umax=65535,var_off=(0x0; 0xffff))
R8_w=map_value(off=0,ks=4,vs=208,imm=0)
131: (67) r1 <<= 48 ; frame1:
R1_w=scalar(smax=9223090561878065152,umax=18446462598732840960,var_off=
(0x0; 0xffff000000000000),s32_min=0,s32_max=0,u32_max=0)
132: (c7) r1 s>>= 48 ; frame1: R1_w=scalar(smin=-
32768,smax=32767)
; if (arg_num >= BPF_USDT_MAX_ARG_CNT || arg_num >= spec->arg_cnt)
133: (79) r2 = *(u64 *)(r10 -24) ; frame1:
R2=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
134: (bd) if r1 <= r2 goto pc+78 ; frame1: R1=scalar(smin=-
32768,smax=32767) R2=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
; arg_spec = &spec->args[arg_num];
135: (79) r1 = *(u64 *)(r10 -24) ; frame1:
R1_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
136: (67) r1 <<= 4 ; frame1:
R1_w=scalar(umax=68719476720,var_off=(0x0;
0xffffffff0),s32_max=2147483632,u32_max=-16)
137: (bf) r2 = r8 ; frame1:
R2_w=map_value(off=0,ks=4,vs=208,imm=0)
R8=map_value(off=0,ks=4,vs=208,imm=0)
138: (0f) r2 += r1 ; frame1:
R1_w=scalar(umax=68719476720,var_off=(0x0;
0xffffffff0),s32_max=2147483632,u32_max=-16)
R2_w=map_value(off=0,ks=4,vs=208,umax=68719476720,var_off=(0x0;
0xffffffff0),s32_max=2147483632,u32_max=-16)
; switch (arg_spec->arg_type) {
139: (61) r1 = *(u32 *)(r2 +8)
#128-#129 make sure that *(u64 *)(r10 -24) <= 11, but when #133
loads it again, this constraint is not there. I guess we need to
force flushing r1 to stack? The following helps:
--- a/tools/lib/bpf/usdt.bpf.h
+++ b/tools/lib/bpf/usdt.bpf.h
@@ -130,7 +130,10 @@ int bpf_usdt_arg(struct pt_regs *ctx, __u64
arg_num, long *res)
if (!spec)
return -ESRCH;
- if (arg_num >= BPF_USDT_MAX_ARG_CNT || arg_num >= spec-
>arg_cnt)
+ if (arg_num >= BPF_USDT_MAX_ARG_CNT)
+ return -ENOENT;
+ barrier_var(arg_num);
+ if (arg_num >= spec->arg_cnt)
return -ENOENT;
arg_spec = &spec->args[arg_num];
I can use this in v2 if it looks good.
Btw, I looked at the barrier_var() definition:
#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))
and I'm curious why it's not defined like this:
#define barrier_var(var) asm volatile("" : "+r"(var))
which is a bit simpler?
>
