| Message ID | 20231212131031.3088661-2-menglong8.dong@gmail.com (mailing list archive) |
|---|---|
| State | Changes Requested |
| Delegated to: | BPF |
| Headers | show |
| Series | bpf: support to trace BPF_JNE | expand |
On Tue, 2023-12-12 at 21:10 +0800, Menglong Dong wrote: > We can derive some new information for BPF_JNE in regs_refine_cond_op(). > Take following code for example: > > /* The type of "a" is u16 */ > if (a > 0 && a < 100) { > /* the range of the register for a is [0, 99], not [1, 99], > * and will cause the following error: > * > * invalid zero-sized read > * > * as a can be 0. > */ > bpf_skb_store_bytes(skb, xx, xx, a, 0); > } > > In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the > TRUE branch, the dst_reg will be marked as known to 0. However, in the > fallthrough(FALSE) branch, the dst_reg will not be handled, which makes > the [min, max] for a is [0, 99], not [1, 99]. > > For BPF_JNE, we can reduce the range of the dst reg if the src reg is a > const and is exactly the edge of the dst reg. > > Signed-off-by: Menglong Dong <menglong8.dong@gmail.com> > --- Acked-by: Eduard Zingerman <eddyz87@gmail.com> > kernel/bpf/verifier.c | 29 ++++++++++++++++++++++++++++- > 1 file changed, 28 insertions(+), 1 deletion(-) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 727a59e4a647..08ee0e02df96 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -14332,7 +14332,34 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state > } > break; > case BPF_JNE: > - /* we don't derive any new information for inequality yet */ > + if (!is_reg_const(reg2, is_jmp32)) > + swap(reg1, reg2); > + if (!is_reg_const(reg2, is_jmp32)) > + break; > + > + /* try to recompute the bound of reg1 if reg2 is a const and > + * is exactly the edge of reg1. > + */ > + val = reg_const_value(reg2, is_jmp32); > + if (is_jmp32) { > + if (reg1->u32_min_value == (u32)val) > + reg1->u32_min_value++; Nit: I spent an unreasonable amount of time trying to figure out if overflow might be an issue here. Would it be helpful to add a comment like below? (not sure, maybe it's obvious and I'm being slow) /* u32_min_value is not equal to 0xffffffff at this point, * because otherwise u32_max_value is 0xffffffff as well, * in such a case both reg1 and reg2 would be constants, * jump would be predicted and reg_set_min_max() won't * be called. * Same reasoning works for all {u,s}{min,max}{32,64} cases below. */
On Wed, Dec 13, 2023 at 7:23 AM Eduard Zingerman <eddyz87@gmail.com> wrote: > > On Tue, 2023-12-12 at 21:10 +0800, Menglong Dong wrote: > > We can derive some new information for BPF_JNE in regs_refine_cond_op(). > > Take following code for example: > > > > /* The type of "a" is u16 */ > > if (a > 0 && a < 100) { > > /* the range of the register for a is [0, 99], not [1, 99], > > * and will cause the following error: > > * > > * invalid zero-sized read > > * > > * as a can be 0. > > */ > > bpf_skb_store_bytes(skb, xx, xx, a, 0); > > } > > > > In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the > > TRUE branch, the dst_reg will be marked as known to 0. However, in the > > fallthrough(FALSE) branch, the dst_reg will not be handled, which makes > > the [min, max] for a is [0, 99], not [1, 99]. > > > > For BPF_JNE, we can reduce the range of the dst reg if the src reg is a > > const and is exactly the edge of the dst reg. > > > > Signed-off-by: Menglong Dong <menglong8.dong@gmail.com> > > --- > > Acked-by: Eduard Zingerman <eddyz87@gmail.com> > > > kernel/bpf/verifier.c | 29 ++++++++++++++++++++++++++++- > > 1 file changed, 28 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 727a59e4a647..08ee0e02df96 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -14332,7 +14332,34 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state > > } > > break; > > case BPF_JNE: > > - /* we don't derive any new information for inequality yet */ > > + if (!is_reg_const(reg2, is_jmp32)) > > + swap(reg1, reg2); > > + if (!is_reg_const(reg2, is_jmp32)) > > + break; > > + > > + /* try to recompute the bound of reg1 if reg2 is a const and > > + * is exactly the edge of reg1. > > + */ > > + val = reg_const_value(reg2, is_jmp32); > > + if (is_jmp32) { > > + if (reg1->u32_min_value == (u32)val) > > + reg1->u32_min_value++; > > Nit: I spent an unreasonable amount of time trying to figure out if > overflow might be an issue here. Would it be helpful to add a > comment like below? (not sure, maybe it's obvious and I'm being slow) > > /* u32_min_value is not equal to 0xffffffff at this point, > * because otherwise u32_max_value is 0xffffffff as well, > * in such a case both reg1 and reg2 would be constants, > * jump would be predicted and reg_set_min_max() won't > * be called. > * Same reasoning works for all {u,s}{min,max}{32,64} cases below. > */ Okay, I'll add this comment in the next version. Thanks! Menglong Dong
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 727a59e4a647..08ee0e02df96 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -14332,7 +14332,34 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state } break; case BPF_JNE: - /* we don't derive any new information for inequality yet */ + if (!is_reg_const(reg2, is_jmp32)) + swap(reg1, reg2); + if (!is_reg_const(reg2, is_jmp32)) + break; + + /* try to recompute the bound of reg1 if reg2 is a const and + * is exactly the edge of reg1. + */ + val = reg_const_value(reg2, is_jmp32); + if (is_jmp32) { + if (reg1->u32_min_value == (u32)val) + reg1->u32_min_value++; + if (reg1->u32_max_value == (u32)val) + reg1->u32_max_value--; + if (reg1->s32_min_value == (s32)val) + reg1->s32_min_value++; + if (reg1->s32_max_value == (s32)val) + reg1->s32_max_value--; + } else { + if (reg1->umin_value == (u64)val) + reg1->umin_value++; + if (reg1->umax_value == (u64)val) + reg1->umax_value--; + if (reg1->smin_value == (s64)val) + reg1->smin_value++; + if (reg1->smax_value == (s64)val) + reg1->smax_value--; + } break; case BPF_JSET: if (!is_reg_const(reg2, is_jmp32))
We can derive some new information for BPF_JNE in regs_refine_cond_op(). Take following code for example: /* The type of "a" is u16 */ if (a > 0 && a < 100) { /* the range of the register for a is [0, 99], not [1, 99], * and will cause the following error: * * invalid zero-sized read * * as a can be 0. */ bpf_skb_store_bytes(skb, xx, xx, a, 0); } In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the TRUE branch, the dst_reg will be marked as known to 0. However, in the fallthrough(FALSE) branch, the dst_reg will not be handled, which makes the [min, max] for a is [0, 99], not [1, 99]. For BPF_JNE, we can reduce the range of the dst reg if the src reg is a const and is exactly the edge of the dst reg. Signed-off-by: Menglong Dong <menglong8.dong@gmail.com> --- kernel/bpf/verifier.c | 29 ++++++++++++++++++++++++++++- 1 file changed, 28 insertions(+), 1 deletion(-)