commit 4c5de127598e1 ("bpf: Emit explicit NULL pointer checks
for PROBE_LDX instructions.") is emitting couple of instructions
before actual load. Consider those additional instructions while
calculating extable offset.
Fixes: 4c5de127598e1 ("bpf: Emit explicit NULL pointer checks for PROBE_LDX instructions.")
Signed-off-by: Ravi Bangoria <[email protected]>
---
arch/x86/net/bpf_jit_comp.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 2a2e290fa5d8..231a8178cc11 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1297,7 +1297,7 @@ st: if (is_imm8(insn->off))
emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
struct exception_table_entry *ex;
- u8 *_insn = image + proglen;
+ u8 *_insn = image + proglen + (u8)(start_of_ldx - temp);
s64 delta;
/* populate jmp_offset for JMP above */
--
2.30.2
On Tue, Jun 22, 2021 at 4:01 AM Ravi Bangoria
<[email protected]> wrote:
>
> commit 4c5de127598e1 ("bpf: Emit explicit NULL pointer checks
> for PROBE_LDX instructions.") is emitting couple of instructions
> before actual load. Consider those additional instructions while
> calculating extable offset.
>
> Fixes: 4c5de127598e1 ("bpf: Emit explicit NULL pointer checks for PROBE_LDX instructions.")
> Signed-off-by: Ravi Bangoria <[email protected]>
> ---
> arch/x86/net/bpf_jit_comp.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2a2e290fa5d8..231a8178cc11 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1297,7 +1297,7 @@ st: if (is_imm8(insn->off))
> emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
> if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
> struct exception_table_entry *ex;
> - u8 *_insn = image + proglen;
> + u8 *_insn = image + proglen + (u8)(start_of_ldx - temp);
Great debugging and the fix. Thanks a lot.
I've dropped (u8) cast, kept (), and applied to bpf tree.
I think it looks cleaner without that cast.
Could you send a followup patch with a selftest, so I don't make
the same mistake again ? ;)
On 6/25/21 9:31 AM, Alexei Starovoitov wrote:
> On Tue, Jun 22, 2021 at 4:01 AM Ravi Bangoria
> <[email protected]> wrote:
>>
>> commit 4c5de127598e1 ("bpf: Emit explicit NULL pointer checks
>> for PROBE_LDX instructions.") is emitting couple of instructions
>> before actual load. Consider those additional instructions while
>> calculating extable offset.
>>
>> Fixes: 4c5de127598e1 ("bpf: Emit explicit NULL pointer checks for PROBE_LDX instructions.")
>> Signed-off-by: Ravi Bangoria <[email protected]>
>> ---
>> arch/x86/net/bpf_jit_comp.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index 2a2e290fa5d8..231a8178cc11 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -1297,7 +1297,7 @@ st: if (is_imm8(insn->off))
>> emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
>> if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
>> struct exception_table_entry *ex;
>> - u8 *_insn = image + proglen;
>> + u8 *_insn = image + proglen + (u8)(start_of_ldx - temp);
>
> Great debugging and the fix. Thanks a lot.
> I've dropped (u8) cast, kept (), and applied to bpf tree.
> I think it looks cleaner without that cast.
Thanks.
> Could you send a followup patch with a selftest, so I don't make
> the same mistake again ? ;)
Unfortunately extable gets involved only for bad kernel pointers and
ideally there should not be any bad pointer in kernel. So there is no
easy way to create a proper selftest for this.
Ravi
On Thu, Jun 24, 2021 at 11:22 PM Ravi Bangoria
<[email protected]> wrote:
> > Could you send a followup patch with a selftest, so I don't make
> > the same mistake again ? ;)
>
> Unfortunately extable gets involved only for bad kernel pointers and
> ideally there should not be any bad pointer in kernel. So there is no
> easy way to create a proper selftest for this.
Right. We have lib/test_bpf.c kernel module for such cases.