On Wed, 2023-07-26 at 23:03 +0300, Eduard Zingerman wrote:
[...]
> > > It looks like `PT_REGS_IP_CORE` macro should not be defined through
> > > bpf_probe_read_kernel(). I'll dig through commit history tomorrow to
> > > understand why is it defined like that now.
> > > help
> >
> > If I recall the rationale was to allow the macros to work for both
> > BPF programs that can do direct dereference (fentry, fexit, tp_btf etc)
> > and for kprobe-style that need to use bpf_probe_read_kernel().
> > Not sure if it would be worth having variants that are purely
> > dereference-based, since we can just use PT_REGS_IP() due to
> > the __builtin_preserve_access_index attributes applied in vmlinux.h.
>
> Sorry, need a bit more time, thanks for the context.
The PT_REGS_*_CORE macros were added by Andrii Nakryiko in [1].
Stated intent there is to use those macros for raw tracepoint
programs. Such programs have `struct pt_regs` as a parameter.
Contexts of type `struct pt_regs` are *not* subject to rewrite by
convert_ctx_access(), so it is valid to use PT_REGS_*_CORE for such
programs.
However, `struct pt_regs` is also a part of `struct
bpf_perf_event_data`. Latter is used as a context parameter for
"perf_event" programs and is a subject to rewrite by
convert_ctx_access(). Thus, PT_REGS_*_CORE macros can't be used for
such programs (because these macro are implemented through
bpf_probe_read_kernel() of which convert_ctx_access() is not aware).
If `struct pt_regs` is defined with `preserve_access_index` attribute
CO-RE relocations are generated for both PT_REGS_IP_CORE and
PT_REGS_IP invocations. So, there is no real need to use *_CORE
variants in combination with `struct bpf_perf_event_data` to have all
CO-RE benefits, e.g.:
$ cat bpf.c
#include "vmlinux.h"
// ...
SEC("perf_event")
int do_test(struct bpf_perf_event_data *ctx) {
return PT_REGS_IP(&ctx->regs);
}
// ...
$ llvm-objdump --no-show-raw-insn -rd bpf.o
...
0000000000000000 <do_test>:
0: r0 = *(u64 *)(r1 + 0x80)
0000000000000000: CO-RE <byte_off> [11] struct bpf_perf_event_data::regs.ip (0:0:16)
1: exit
[1] b8ebce86ffe6 ("libbpf: Provide CO-RE variants of PT_REGS macros")
---
I think the following should be done:
- Timofei's code should use PT_REGS_IP and make sure that `struct
pt_regs` has preserve_access_index annotation (e.g. use vmlinux.h);
- verifier should be adjusted to report error when
bpf_probe_read_kernel() (and similar) are used to read from "fake"
contexts.
- (maybe?) update PT_REGS_*_CORE to use `__builtin_preserve_access_index`
(to allow usage with `bpf_perf_event_data` context).
[...]
