bpf: Preserve boundaries and track scalars on narrowing fill

When the width of a fill is smaller than the width of the preceding
spill, the information about scalar boundaries can still be preserved,
as long as it's coerced to the right width (done by coerce_reg_to_size).
Even further, if the actual value fits into the fill width, the ID can
be preserved as well for further tracking of equal scalars.

Implement the above improvements, which makes narrowing fills behave the
same as narrowing spills and MOVs between registers.

Two tests are adjusted to accommodate for endianness differences and to
take into account that it's now allowed to do a narrowing fill from the
least significant bits.

reg_bounds_sync is added to coerce_reg_to_size to correctly adjust
umin/umax boundaries after the var_off truncation, for example, a 64-bit
value 0xXXXXXXXX00000000, when read as a 32-bit, gets umin = 0, umax =
0xFFFFFFFF, var_off = (0x0; 0xffffffff00000000), which needs to be
synced down to umax = 0, otherwise reg_bounds_sanity_check doesn't pass.

Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240127175237.526726-4-maxtram95@gmail.com

1 files changed, 9 insertions, 0 deletions

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 0dcde339dc7e..84365e6dd85d 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -919,6 +919,15 @@ static inline void mark_verifier_state_scratched(struct bpf_verifier_env *env)
 	env->scratched_stack_slots = ~0ULL;
 }
 
+static inline bool bpf_stack_narrow_access_ok(int off, int fill_size, int spill_size)
+{
+#ifdef __BIG_ENDIAN
+	off -= spill_size - fill_size;
+#endif
+
+	return !(off % BPF_REG_SIZE);
+}
+
 const char *reg_type_str(struct bpf_verifier_env *env, enum bpf_reg_type type);
 const char *dynptr_type_str(enum bpf_dynptr_type type);
 const char *iter_type_str(const struct btf *btf, u32 btf_id);