On 4/18/18 9:35 PM, Alexei Starovoitov wrote:
On Wed, Apr 18, 2018 at 09:54:39AM -0700, Yonghong Song wrote:When helpers like bpf_get_stack returns an int value and later on used for arithmetic computation, the LSH and ARSH operations are often required to get proper sign extension into 64-bit. For example, without this patch: 54: R0=inv(id=0,umax_value=800) 54: (bf) r8 = r0 55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800) 55: (67) r8 <<= 32 56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000)) 56: (c7) r8 s>>= 32 57: R8=inv(id=0) With this patch: 54: R0=inv(id=0,umax_value=800) 54: (bf) r8 = r0 55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800) 55: (67) r8 <<= 32 56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000)) 56: (c7) r8 s>>= 32 57: R8=inv(id=0, umax_value=800,var_off=(0x0; 0x3ff)) With better range of "R8", later on when "R8" is added to other register, e.g., a map pointer or scalar-value register, the better register range can be derived and verifier failure may be avoided. In our later example, ...... usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK); if (usize < 0) return 0; ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0); ...... Without improving ARSH value range tracking, the register representing "max_len - usize" will have smin_value equal to S64_MIN and will be rejected by verifier. Signed-off-by: Yonghong Song <y...@fb.com> --- kernel/bpf/verifier.c | 1 + 1 file changed, 1 insertion(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index a8302c3..6148d31 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2944,6 +2944,7 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, __update_reg_bounds(dst_reg); break; case BPF_RSH: + case BPF_ARSH:I don't think that's correct. The code further down is very RSH specific.
Okay, I may need to introduce tnum_arshift then.
if (umax_val >= insn_bitness) { /* Shifts greater than 31 or 63 are undefined. * This includes shifts by a negative number. -- 2.9.5
