On Tue, 2023-07-25 at 15:04 +0100, Alan Maguire wrote:
> On 25/07/2023 00:00, Alan Maguire wrote:
> > 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.
> >
>
>
> The problem occurs with latest bpf-next + latest LLVM too. Perf event
> programs fix up context accesses to the "struct bpf_perf_event_data *"
> context, so accessing ctx->regs in your program becomes accessing the
> "struct bpf_perf_event_data_kern *" regs, which is a pointer to
> struct pt_regs. So I _think_ that's why the
>
> __builtin_preserve_access_index(({
> p_core = ctx->regs.ip;
> }));
>
>
> ...works; ctx->regs is fixed up to point at the right place, then
> CO-RE does its thing with the results. Contrast this with
>
> bpf_probe_read_kernel(&ip, sizeof(ip), &ctx->regs.ip);
>
> In the latter case, the fixups don't seem to happen and we get a
> bogus address which appears to be consistently 218 bytes after the ctx
> pointer. I've confirmed that a basic bpf_probe_read_kernel()
> exposes the issue (and gives the same wrong address as a CO-RE-wrapped
> bpf_probe_read_kernel()).
>
> I tried some permutations like defining
>
> struct pt_regs *regs = &ctx->regs;
>
> ...to see if that helps, but I think in that case the accesses aren't
> caught by the verifier because we use the & operator on the ctx->regs.
>
> Not sure how smart the verifier can be about context accesses like this;
> can someone who understands that code better than me take a look at this?
Hi Alan,
Your analysis is correct: verifier applies rewrites to instructions
that read/write from/to certain context fields, including
`struct bpf_perf_event_data`.
This is done by function verifier.c:convert_ctx_accesses().
This function handles BPF_LDX, BPF_STX and BPF_ST instructions, but it
does not handle calls to helpers like bpf_probe_read_kernel().
So, when code generated for PT_REGS_IP(&ctx->regs) is processed, the
correct access sequence is inserted by function
bpf_trace.c:pe_prog_convert_ctx_access() (see below).
But code generated for `PT_REGS_IP_CORE(&ctx->regs)` is not modified.
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.
Thanks,
Eduard
---
Below is annotated example, inpatient reader might skip it
For the following test program:
#include "vmlinux.h"
...
SEC("perf_event")
int do_test(struct bpf_perf_event_data *ctx) {
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;
}
Generated BPF code looks as follows:
$ llvm-objdump --no-show-raw-insn -rd bpf.linked.o
...
0000000000000000 <do_test>:
# Third argument for bpf_probe_read_kernel: offset of bpf_perf_event_data::regs.ip
0: r2 = 0x80
0000000000000000: CO-RE <byte_off> [2] struct bpf_perf_event_data::regs.ip (0:0:16)
1: r3 = r1
2: r3 += r2
# The "non CO-RE" version of PT_REGS_IP is, in fact, CO-RE
# because `struct bpf_perf_event_data` has preserve_access_index
# tag in the vmlinux.h.
# Here the regs.ip is stored in r6 to be used after the call
# to bpf_probe_read_kernel() (from PT_REGS_IP_CORE).
3: r6 = *(u64 *)(r1 + 0x80)
0000000000000018: CO-RE <byte_off> [2] struct bpf_perf_event_data::regs.ip (0:0:16)
# First argument for bpf_probe_read_kernel: a place on stack to put read result to.
4: r1 = r10
5: r1 += -0x8
# Second argument for bpf_probe_read_kernel: the size of the field to read.
6: w2 = 0x8
# Call to bpf_probe_read_kernel()
7: call 0x71
# Fourth parameter of bpf_printk: p_core read from stack
# (was written by call to bpf_probe_read_kernel)
8: r4 = *(u64 *)(r10 - 0x8)
# First parameter of bpf_printk: control string
9: r1 = 0x0 ll
0000000000000048: R_BPF_64_64 .rodata
# Second parameter of bpf_printk: size of the control string
11: w2 = 0x1b
# Third parameter of bpf_printk: p (see addr 3)
12: r3 = r6
# Call to bpf_printk
13: call 0x6
; return 0;
14: w0 = 0x0
15: exit
I get the following BPF after all verifier rewrites are applied
(including verifier.c:convert_ctx_accesses()):
# ./tools/bpf/bpftool/bpftool prog dump xlated id 114
int do_test(struct bpf_perf_event_data * ctx):
; int do_test(struct bpf_perf_event_data *ctx) {
0: (b7) r2 = 128 | CO-RE replacement, 128 is a valid offset for
| bpf_perf_event_data::regs.ip in my kernel
1: (bf) r3 = r1
2: (0f) r3 += r2
3: (79) r6 = *(u64 *)(r1 +0) | This is an expantion of the
4: (79) r6 = *(u64 *)(r6 +128) | r6 = *(u64 *)(r1 + 0x80)
5: (bf) r1 = r10 | Created by bpf_trace.c:pe_prog_convert_ctx_access()
6: (07) r1 += -8
7: (b4) w2 = 8
8: (85) call bpf_probe_read_kernel#-91984
9: (79) r4 = *(u64 *)(r10 -8)
10: (18) r1 = map[id:59][0]+0
12: (b4) w2 = 27
13: (bf) r3 = r6
14: (85) call bpf_trace_printk#-85520
15: (b4) w0 = 0
16: (95) exit
