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>
---
include/linux/tnum.h | 4 +++-
kernel/bpf/tnum.c | 10 ++++++++++
kernel/bpf/verifier.c | 41 +++++++++++++++++++++++++++++++++++++++++
3 files changed, 54 insertions(+), 1 deletion(-)
diff --git a/include/linux/tnum.h b/include/linux/tnum.h
index 0d2d3da..c7dc2b5 100644
--- a/include/linux/tnum.h
+++ b/include/linux/tnum.h
@@ -23,8 +23,10 @@ struct tnum tnum_range(u64 min, u64 max);
/* Arithmetic and logical ops */
/* Shift a tnum left (by a fixed shift) */
struct tnum tnum_lshift(struct tnum a, u8 shift);
-/* Shift a tnum right (by a fixed shift) */
+/* Shift (rsh) a tnum right (by a fixed shift) */
struct tnum tnum_rshift(struct tnum a, u8 shift);
+/* Shift (arsh) a tnum right (by a fixed min_shift) */
+struct tnum tnum_arshift(struct tnum a, u8 min_shift);
/* Add two tnums, return @a + @b */
struct tnum tnum_add(struct tnum a, struct tnum b);
/* Subtract two tnums, return @a - @b */
diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
index 1f4bf68..938d412 100644
--- a/kernel/bpf/tnum.c
+++ b/kernel/bpf/tnum.c
@@ -43,6 +43,16 @@ struct tnum tnum_rshift(struct tnum a, u8 shift)
return TNUM(a.value >> shift, a.mask >> shift);
}
+struct tnum tnum_arshift(struct tnum a, u8 min_shift)
+{
+ /* if a.value is negative, arithmetic shifting by minimum shift
+ * will have larger negative offset compared to more shifting.
+ * If a.value is nonnegative, arithmetic shifting by minimum shift
+ * will have larger positive offset compare to more shifting.
+ */
+ return TNUM((s64)a.value >> min_shift, (s64)a.mask >> min_shift);
+}
+
struct tnum tnum_add(struct tnum a, struct tnum b)
{
u64 sm, sv, sigma, chi, mu;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 217d92a..643923e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2968,6 +2968,47 @@ 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;
+ }
+
+ /* BPF_ARSH is an arithmetic shift. The new range of
+ * smin_value and smax_value should take the sign
+ * into consideration.
+ *
+ * For example, if smin_value = -16, umin_val = 0
+ * and umax_val = 2, the new smin_value should be
+ * -16 >> 0 = -16 since -16 >> 2 = -4.
+ * If smin_value = 16, umin_val = 0 and umax_val = 2,
+ * the new smin_value should be 16 >> 2 = 4.
+ *
+ * Now suppose smax_value = -4, umin_val = 0 and
+ * umax_val = 2, the new smax_value should be
+ * -4 >> 2 = -1. If smax_value = 32 with the same
+ * umin_val/umax_val, the new smax_value should remain 32.
+ */
+ 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;
+ dst_reg->var_off = tnum_arshift(dst_reg->var_off, umin_val);
+
+ /* blow away the dst_reg umin_value/umax_value and rely on
+ * dst_reg var_off to refine the result.
+ */
+ dst_reg->umin_value = 0;
+ dst_reg->umax_value = U64_MAX;
+ __update_reg_bounds(dst_reg);
+ break;
default:
mark_reg_unknown(env, regs, insn->dst_reg);
break;
--
2.9.5
