Re: [PATCH bpf-next v2 07/13] bpf: track low-32 scalar equality across sign-extending movs

[Alexei Starovoitov]

On Thu Sep 10, 2026 at 9:46 AM PDT, Vineet Gupta wrote:
> The zero-extending mov records that dst shares src's low 32 bits. A 32-bit
> sign extension shares them too -- it keeps the low half and fills the high
> half from bit 31 -- so the same link applies, with a different rule for
> rebuilding the high bits:
>
>   r6 = ...              /* full 64-bit unknown */
>   r7 = (s32)r6          /* 32-bit sign-extending mov */
>   if w6 == -1 goto ...  /* taken: r6's low 32 bits are all ones */
>   ...                   /* r7 is -1, not deduced today */
>
> Add SUBREG_SEXT alongside SUBREG_ZEXT, and sext_32_to_64() alongside
> zext_32_to_64() to drive the reconstruction. Both work from the base's
> 32-bit range, which is what a 32-bit compare narrows.
> coerce_reg_to_size_sx() cannot serve here: it reads smin/smax, which
> straddle after such a compare and collapse to the full field range.
>
> tnum_sext() is the counterpart to tnum_cast(). Unlike a tnum_range() over
> the new bounds it keeps the known low bits.
>
> The enum has room for the third value, so bpf_reg_state stays 80 bytes.
>
> Unlike the zero-extending arm, a self-mov can form a link here, but only
> when src is already linked: r0 = (s32)r0 is how a sign-extended int return
> lands. On an unlinked register there is nothing to link to, and minting an
> id would leave the register describing itself.
>
> Signed-off-by: Vineet Gupta <vineet.gupta@xxxxxxxxx>
> ---
> v2: was RFC 5/6.
>  - no forward declaration (Eduard)
>  - src renamed known_reg (Eduard)
>  - sext_32_to_64() and tnum_sext() rather than reusing
>    coerce_reg_to_size_sx(); the sync path needs the base's 32-bit range,
>    see the cover letter
>  - tnum_sext() keeps the known low bits a tnum_range() would drop (Eduard)
>  - a self-mov links only when src already has an id, narrower than the RFC
>
>  include/linux/bpf_verifier.h |  1 +
>  include/linux/tnum.h         |  3 +++
>  kernel/bpf/log.c             |  2 ++
>  kernel/bpf/tnum.c            | 15 ++++++++++++
>  kernel/bpf/verifier.c        | 47 +++++++++++++++++++++++++++++++++---
>  5 files changed, 65 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index f1b01059c5da..920c9490ecc8 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -53,6 +53,7 @@ enum bpf_add_const {
>  enum bpf_subreg {
>  	SUBREG_NONE = 0,
>  	SUBREG_ZEXT,		/* high bits are zero (32-bit zero-extending mov) */
> +	SUBREG_SEXT,		/* high bits repeat bit 31 (32-bit sign-extending mov) */

If my earlier suggestion to model SUBREG_ZEXT as add_const32 with delta == 0
works, then sign extension could be:

enum bpf_add_const {
       ADD_CONST_NONE = 0,
       ADD_CONST_32,    /* delta was added with a 32-bit ALU op */ /* works as ZERO_EXTEND when delta == 0 */
       ADD_CONST_64,    /* ... with a 64-bit ALU op */
       SIGN_EXTEND_32, 	/* delta has to be == 0 */
};

and if we really want to we can track movsx 8 and 16 too.
Which might be an overkill.