Re: bpf_errno. Was: [PATCH RFC bpf-next 1/3] bpf: report probe fault to BPF stderr

[Menglong Dong <menglong.dong@linux.dev>]

On 2025/10/11 02:55, Eduard Zingerman wrote:
> On Fri, 2025-10-10 at 20:05 +0800, Menglong Dong wrote:
> 
> [...]
> 
> > save errno to r0(Eduard)
> > -----------------------------------
> > Save the errno to r0 in the exception handler of BPF_PROBE_MEM,
> > and read r0 with a __kfun in BPF program. (Not sure if I understand
> > it correctly).
> > 
> > This sounds effective, but won't this break the usage of r0? I mean,
> > the r0 can be used by the BPF program somewhere.
> 
> What I meant is that for cases when someone wants to check for memory
> access error, there is already bpf_probe_read_kernel(). It's return
> value in r0 and is defined for both success and failure cases.
> 
> The problem with it, is that it has a function call overhead.
> But we can workaround that for 1,2,4,8 byte accesses, by replacing
> helper call by some `BPF_LDX | BPF_PROBE_MEM1 | <size>`,
> where BPF_PROBE_MEM1 is different from BPF_PROBE_MEM and tells
> jit that exception handler for this memory access needs to set
> r0 to -EFAULT if it is executed.
> 
> The inconvenient part here is that one can't do chaining,
> like a->b->c, using bpf_probe_read_kernel().
> One needs to insert bpf_probe_read_kernel() call at each step of a
> chain, which is a bit of a pain.  Maybe it can be alleviated using
> some vararg macro.

Thanks for the explication, and I see now.

Interesting idea, and I think this is something that we can do
despite this problem, which can optimize the performance of
bpf_probe_read_kernel().

Thanks!
Menglong Dong

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