[PATCH v5 bpf-next 09/23] bpf: drop knowledge-losing __reg_combine_{32,64}_into_{64,32} logic

Andrii Nakryiko
2023-10-27T18:17:01Z

When performing 32-bit conditional operation operating on lower 32 bits
of a full 64-bit register, register full value isn't changed. We just
potentially gain new knowledge about that register's lower 32 bits.

Unfortunately, __reg_combine_{32,64}_into_{64,32} logic that
reg_set_min_max() performs as a last step, can lose information in some
cases due to __mark_reg64_unbounded() and __reg_assign_32_into_64().
That's bad and completely unnecessary. Especially __reg_assign_32_into_64()
looks completely out of place here, because we are not performing
zero-extending subregister assignment during conditional jump.

So this patch replaced __reg_combine_* with just a normal
reg_bounds_sync() which will do a proper job of deriving u64/s64 bounds
from u32/s32, and vice versa (among all other combinations).

__reg_combine_64_into_32() is also used in one more place,
coerce_reg_to_size(), while handling 1- and 2-byte register loads.
Looking into this, it seems like besides marking subregister as
unbounded before performing reg_bounds_sync(), we were also performing
deduction of smin32/smax32 and umin32/umax32 bounds from respective
smin/smax and umin/umax bounds. It's now redundant as reg_bounds_sync()
performs all the same logic more generically (e.g., without unnecessary
assumption that upper 32 bits of full register should be zero).

Long story short, we remove __reg_combine_64_into_32() completely, and
coerce_reg_to_size() now only does resetting subreg to unbounded and then
performing reg_bounds_sync() to recover as much information as possible
from 64-bit umin/umax and smin/smax bounds, set explicitly in
coerce_reg_to_size() earlier.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/verifier.c | 60 ++++++-------------------------------------
1 file changed, 8 insertions(+), 52 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6b0736c04ebe..f5fcb7fb2c67 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2641,51 +2641,6 @@ static void __reg_assign_32_into_64(struct bpf_reg_state *reg)
}
}

-static void __reg_combine_32_into_64(struct bpf_reg_state *reg)
-{
- /* special case when 64-bit register has upper 32-bit register
- * zeroed. Typically happens after zext or <<32, >>32 sequence
- * allowing us to use 32-bit bounds directly,
- */
- if (tnum_equals_const(tnum_clear_subreg(reg->var_off), 0)) {
- __reg_assign_32_into_64(reg);
- } else {
- /* Otherwise the best we can do is push lower 32bit known and
- * unknown bits into register (var_off set from jmp logic)
- * then learn as much as possible from the 64-bit tnum
- * known and unknown bits. The previous smin/smax bounds are
- * invalid here because of jmp32 compare so mark them unknown
- * so they do not impact tnum bounds calculation.
- */
- __mark_reg64_unbounded(reg);
- }
- reg_bounds_sync(reg);
-}
-
-static bool __reg64_bound_s32(s64 a)
-{
- return a >= S32_MIN && a <= S32_MAX;
-}
-
-static bool __reg64_bound_u32(u64 a)
-{
- return a >= U32_MIN && a <= U32_MAX;
-}
-
-static void __reg_combine_64_into_32(struct bpf_reg_state *reg)
-{
- __mark_reg32_unbounded(reg);
- if (__reg64_bound_s32(reg->smin_value) && __reg64_bound_s32(reg->smax_value)) {
- reg->s32_min_value = (s32)reg->smin_value;
- reg->s32_max_value = (s32)reg->smax_value;
- }
- if (__reg64_bound_u32(reg->umin_value) && __reg64_bound_u32(reg->umax_value)) {
- reg->u32_min_value = (u32)reg->umin_value;
- reg->u32_max_value = (u32)reg->umax_value;
- }
- reg_bounds_sync(reg);
-}
-
/* Mark a register as having a completely unknown (scalar) value. */
static void __mark_reg_unknown(const struct bpf_verifier_env *env,
struct bpf_reg_state *reg)
@@ -6382,9 +6337,10 @@ static void coerce_reg_to_size(struct bpf_reg_state *reg, int size)
* values are also truncated so we push 64-bit bounds into
* 32-bit bounds. Above were truncated < 32-bits already.
*/
- if (size >= 4)
- return;
- __reg_combine_64_into_32(reg);
+ if (size < 4) {
+ __mark_reg32_unbounded(reg);
+ reg_bounds_sync(reg);
+ }
}

static void set_sext64_default_val(struct bpf_reg_state *reg, int size)
@@ -14623,13 +14579,13 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg,
tnum_subreg(false_32off));
true_reg->var_off = tnum_or(tnum_clear_subreg(true_64off),
tnum_subreg(true_32off));
- __reg_combine_32_into_64(false_reg);
- __reg_combine_32_into_64(true_reg);
+ reg_bounds_sync(false_reg);
+ reg_bounds_sync(true_reg);
} else {
false_reg->var_off = false_64off;
true_reg->var_off = true_64off;
- __reg_combine_64_into_32(false_reg);
- __reg_combine_64_into_32(true_reg);
+ reg_bounds_sync(false_reg);
+ reg_bounds_sync(true_reg);
}
}

--
2.34.1