On 24/07/2023 16:04, Timofei Pushkin wrote:
> On Mon, Jul 24, 2023 at 3:36 PM Alan Maguire <alan.maguire@xxxxxxxxxx> wrote:
>>
>> On 24/07/2023 11:32, Timofei Pushkin wrote:
>>> Dear BPF community,
>>>
>>> I'm developing a perf_event BPF program which reads some register
>>> values (frame and instruction pointers in particular) from the context
>>> provided to it. I found that CO-RE-enabled PT_REGS macros give results
>>> different from the results of the usual PT_REGS macros. I run the
>>> program on the same system I compiled it on, and so I cannot
>>> understand why the results differ and which ones should I use?
>>>
>>> From my tests, the results of the usual macros are the correct ones
>>> (e.g. I can symbolize the instruction pointers I get this way), but
>>> since I try to follow the CO-RE principle, it seems like I should be
>>> using the CO-RE-enabled variants instead.
>>>
>>> I did some experiments and found out that it is the
>>> bpf_probe_read_kernel part of the CO-RE-enabled PT_REGS macros that
>>> change the results and not __builtin_preserve_access_index. But I
>>> still don't get why exactly it changes the results.
>>>
>>
>> Can you provide the exact usage of the BPF CO-RE macros that isn't
>> working, and the equivalent non-CO-RE version that is? Also if you
>
> As a minimal example, I wrote the following little BPF program which
> prints instruction pointers obtained with non-CO-RE and CO-RE macros:
>
> volatile const pid_t target_pid;
>
> SEC("perf_event")
> int do_test(struct bpf_perf_event_data *ctx) {
> pid_t pid = bpf_get_current_pid_tgid();
> if (pid != target_pid) return 0;
>
> unsigned long p = PT_REGS_IP(&ctx->regs);
> unsigned long p_core = PT_REGS_IP_CORE(&ctx->regs);
> bpf_printk("non-CO-RE: %lx, CO-RE: %lx", p, p_core);
>
> return 0;
> }
>
> From user space, I set the target PID and attach the program to CPU
> clock perf events (error checking and cleanup omitted for brevity):
>
> int main(int argc, char *argv[]) {
> // Load the program also setting the target PID
> struct test_program_bpf *skel = test_program_bpf__open();
> skel->rodata->target_pid = (pid_t) strtol(argv[1], NULL, 10);
> test_program_bpf__load(skel);
>
> // Attach to perf events
> struct perf_event_attr attr = {
> .type = PERF_TYPE_SOFTWARE,
> .size = sizeof(struct perf_event_attr),
> .config = PERF_COUNT_SW_CPU_CLOCK,
> .sample_freq = 1,
> .freq = true
> };
> for (int cpu_i = 0; cpu_i < libbpf_num_possible_cpus(); cpu_i++) {
> int perf_fd = syscall(SYS_perf_event_open, &attr, -1, cpu_i, -1, 0);
> bpf_program__attach_perf_event(skel->progs.do_test, perf_fd);
> }
>
> // Wait for Ctrl-C
> pause();
> return 0;
> }
>
> As an experiment, I launched a simple C program with an endless loop
> in main and started the BPF program above with its target PID set to
> the PID of this simple C program. Then by checking the virtual memory
> mapped for the C program (with "cat /proc/<PID>/maps"), I found out
> that its .text section got mapped into 55ca2577b000-55ca2577c000
> address space. When I checked the output of the BPF program, I got
> "non-CO-RE: 55ca2577b131, CO-RE: ffffa58810527e48". As you can see,
> the non-CO-RE result maps into the .text section of the launched C
> program (as it should since this is the value of the instruction
> pointer), while the CO-RE result does not.
>
> Alternatively, if I replace PT_REGS_IP and PT_REGS_IP_CORE with the
> equivalents for the stack pointer (PT_REGS_SP and PT_REGS_SP_CORE), I
> get results that correspond to the stack address space from the
> non-CO-RE macro, but I always get 0 from the CO-RE macro.
>
>> can provide details on the platform you're running on that will
>> help narrow down the issue. Thanks!
>
> Sure. I'm running Ubuntu 22.04.1, kernel version 5.19.0-46-generic,
> the architecture is x86_64, clang 14.0.0 is used to compile BPF
> programs with flags -g -O2 -D__TARGET_ARCH_x86.
>
Thanks for the additional details! I've reproduced this on
bpf-next with LLVM 15; I'm seeing the same issues with the CO-RE
macros, and with BPF_CORE_READ(). However with extra libbpf debugging
I do see that we pick up the right type id/index for the ip field in
pt_regs:
libbpf: prog 'do_test': relo #4: matching candidate #0 <byte_off> [216]
struct pt_regs.ip (0:16 @ offset 128)
One thing I noticed - perhaps this will ring some bells for someone -
if I use __builtin_preserve_access_index() I get the same (correct)
value for ip as is retrieved with PT_REGS_IP():
__builtin_preserve_access_index(({
p_core = ctx->regs.ip;
}));
I'll check with latest LLVM to see if the issue persists there.
Alan
> Thanks,
> Timofei
>
>>
>> Alan
>>
>>> Thank you in advance,
>>> Timofei
>>>
>
