On Wed, Dec 20, 2023 at 11:40:10PM +0200, Maxim Mikityanskiy wrote:
> 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@xxxxxxxxxxxxx>
> ---
> kernel/bpf/verifier.c | 20 ++++++++++---
> .../selftests/bpf/progs/verifier_spill_fill.c | 28 +++++++++++++------
> 2 files changed, 35 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9b5053389739..b6e252539e52 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4772,7 +4772,13 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
> if (dst_regno < 0)
> return 0;
>
> - if (!(off % BPF_REG_SIZE) && size == spill_size) {
> + if (size <= spill_size &&
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> + !(off % BPF_REG_SIZE)
> +#else
> + !((off + size - spill_size) % BPF_REG_SIZE)
> +#endif
If I understand correctly, it is preferred to keep endianess checking
macro out of verfier.c and have helper function handle them instead.
E.g. See bpf_ctx_narrow_access_offset() from include/linux/filter.h
> [...]
