On 9/12/26 12:02 PM, Alexei Starovoitov wrote:
On Thu Sep 10, 2026 at 9:46 AM PDT, Vineet Gupta wrote:
coerce_reg_to_size_sx() decides whether a sign extension is lossless by
comparing the bits above the field in smin and smax:
top_smax_value = ((u64)reg_smax(reg) >> num_bits) << num_bits;
top_smin_value = ((u64)reg_smin(reg) >> num_bits) << num_bits;
if (top_smax_value != top_smin_value)
goto out;
Equal high bits do imply the truncation is lossless, but the converse does
not hold. Whenever the range straddles zero the high bits necessarily
differ -- smin sign-extends to all ones, smax to all zeroes -- even when
every value in the range fits the field and (sN)v == v throughout. The
second gate, "both of s64_max/s64_min positive or negative", rejects the
same shape again for the same reason.
So a register holding an errno-or-zero value, [-4095, 0], comes out of r0
= (s32)r0 as the full [S32_MIN, S32_MAX] even though the instruction is a
no-op on it. The no_sext test at the call site does not help: it is an
unsigned check, so it only covers non-negative values that fit.
Test the range against the field directly and return early when it fits.
Sign extension is then the identity, so nothing needs updating -- which
also preserves var_off, where the existing path would have replaced known
bits with a coarse tnum_range().
This only tightens: the early return fires exactly where the value is
provably unchanged, and the cases the current tests do accept still take
the same path and produce the same bounds.
Signed-off-by: Vineet Gupta <[email protected]>
---
v2: new. coerce_reg_to_size_sx() is fixed rather than special-cased,
which is what was asked on RFC 5/6; it removes the RFC's
call-then-overwrite at the mov site.
kernel/bpf/verifier.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 58e788f53ae5..eb093194e2a3 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5754,6 +5754,7 @@ static void set_sext64_default_val(struct bpf_reg_state
*reg, int size)
static void coerce_reg_to_size_sx(struct bpf_reg_state *reg, int size)
{
s64 init_s64_max, init_s64_min, s64_max, s64_min, u64_cval;
+ s64 field_smin, field_smax;
u64 top_smax_value, top_smin_value;
u64 num_bits = size * 8;
@@ -5773,6 +5774,27 @@ static void coerce_reg_to_size_sx(struct bpf_reg_state *reg, int size)
return;
}
+ if (size == 1) {
+ field_smin = S8_MIN;
+ field_smax = S8_MAX;
+ } else if (size == 2) {
+ field_smin = S16_MIN;
+ field_smax = S16_MAX;
+ } else {
+ /* size == 4 */
+ field_smin = S32_MIN;
+ field_smax = S32_MAX;
+ }
+
+ /*
+ * The range already fits the field, so (sN)v == v for every value the
+ * register can hold and the sign extension changes nothing. The tests
+ * below cannot reach this case once smin is negative: a negative smin
+ * and a non-negative smax never share their high bits.
+ */
+ if (reg_smin(reg) >= field_smin && reg_smax(reg) <= field_smax)
+ return;
+
This patch can be a patch 1, since it looks unrelated to the rest?
Would be good to test it individually. veristat run before/after.
OK, and the corresponding patch moves too.
Thx,
-Vineet
