On 4/22/18 9:19 PM, Alexei Starovoitov wrote:
On Sun, Apr 22, 2018 at 07:49:13PM -0700, Yonghong Song wrote:
On 4/22/18 5:16 PM, Alexei Starovoitov wrote:
On Fri, Apr 20, 2018 at 03:18:37PM -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 | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3c8bb92..01c215d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2975,6 +2975,32 @@ static int adjust_scalar_min_max_vals(struct
bpf_verifier_env *env,
/* We may learn something more from the var_off */
__update_reg_bounds(dst_reg);
break;
+ case BPF_ARSH:
+ if (umax_val >= insn_bitness) {
+ /* Shifts greater than 31 or 63 are undefined.
+ * This includes shifts by a negative number.
+ */
+ mark_reg_unknown(env, regs, insn->dst_reg);
+ break;
+ }
+ if (dst_reg->smin_value < 0)
+ dst_reg->smin_value >>= umin_val;
+ else
+ dst_reg->smin_value >>= umax_val;
+ if (dst_reg->smax_value < 0)
+ dst_reg->smax_value >>= umax_val;
+ else
+ dst_reg->smax_value >>= umin_val;
+ if (src_known)
+ dst_reg->var_off = tnum_rshift(dst_reg->var_off,
+ umin_val);
+ else
+ dst_reg->var_off = tnum_rshift(tnum_unknown, umin_val);
+ dst_reg->umin_value >>= umax_val;
+ dst_reg->umax_value >>= umin_val;
+ /* We may learn something more from the var_off */
+ __update_reg_bounds(dst_reg);
I'm struggling to understand how these bounds are computed.
Could you add examples in the comments?
Okay, let me try to add some comments for better understanding.
In particular if dst_reg is unknown (tnum.mask == -1)
the above tnum_rshift() will clear upper bits and will make it
64-bit positive, but that doesn't seem correct.
What am I missing?
Considering this is arith shift, we probably should just have
dst_reg->var_off = tnum_unknown to be conservative.
I could miss something here as well. Let me try to write more
detailed explanation, hopefully to cover all corner cases.
Is there a use case for !src_known ?
For typical bpf programs, the shift amount should always be known...
If src_known is true, it must be dealing custom packets or custom
data structures in tracing, etc.
I think test_verifier should have 100% line coverage of verifier.c
and every 'if' condition in the verifier needs to have real use case
behind it.
It's still on my todo list to get rid of [su][min|max]_value tracking
that was introduced without solid justification.
