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. [...]
