>>
>> Since we're piling on ideas, one of the other things that I think
>> could be useful in general (and maybe should be done orthogonally to
>> bpf_errno)
>> is making some empty nop function and making it not traceable reliably
>> across arches and invoke it in the bpf exception handler.
>
> No new traceable function is needed, since ex_handler_bpf itself can
> already be traced via fentry.
>
> If users really want to detect whether a fault occurred, they could
> attach a program to ex_handler_bpf and record fault events into a map.
> However, this approach would be too heavyweight just to check for a
> simple fault condition.
>
As ex_handler_bpf can already be traced using fentry, a potential
approach without modifying the kernel would be:
1. In the fentry program:
int is_fault SEC(".percpu.fault");
SEC("fentry/ex_handler_bpf")
int BPF_PROG(f__ex, const struct exception_table_entry *x, struct
pt_regs *regs)
{
is_fault = 1;
return 0;
}
2. In the main program:
int is_fault SEC(".percpu.fault");
is_fault = 0;
/* probe read */
if (is_fault)
/* handle fault */;
The main idea is that both programs share the same ".percpu.fault" map,
so the variable 'is_fault' can be accessed from both sides.
Here, ".percpu.fault" represents a percpu_array map section, which is
expected to be supported in the future.
In the meantime, it can simply be replaced with a regular percpu_array map.
Finally, this approach is conceptually similar to the idea of using a
global errno.
Thanks,
Leon
