On Mon, Sep 9, 2024 at 10:34 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Mon, Sep 9, 2024 at 8:43 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
> >
> > Salvatore Benedetto reported an issue that when doing syscall tracepoint
> > tracing the kernel stack is empty. For example, using the following
> > command line
> > bpftrace -e 'tracepoint:syscalls:sys_enter_read { print("Kernel Stack\n"); print(kstack()); }'
> > the output will be
> > ===
> > Kernel Stack
> > ===
> >
> > Further analysis shows that pt_regs used for bpf syscall tracepoint
> > tracing is from the one constructed during user->kernel transition.
> > The call stack looks like
> > perf_syscall_enter+0x88/0x7c0
> > trace_sys_enter+0x41/0x80
> > syscall_trace_enter+0x100/0x160
> > do_syscall_64+0x38/0xf0
> > entry_SYSCALL_64_after_hwframe+0x76/0x7e
> >
> > The ip address stored in pt_regs is from user space hence no kernel
> > stack is printed.
> >
> > To fix the issue, we need to use kernel address from pt_regs.
> > In kernel repo, there are already a few cases like this. For example,
> > in kernel/trace/bpf_trace.c, several perf_fetch_caller_regs(fake_regs_ptr)
> > instances are used to supply ip address or use ip address to construct
> > call stack.
> >
> > The patch follows the above example by using a fake pt_regs.
> > The pt_regs is stored in local stack since the syscall tracepoint
> > tracing is in process context and there are no possibility that
> > different concurrent syscall tracepoint tracing could mess up with each
> > other. This is similar to a perf_fetch_caller_regs() use case in
> > kernel/trace/trace_event_perf.c with function perf_ftrace_function_call()
> > where a local pt_regs is used.
> >
> > With this patch, for the above bpftrace script, I got the following output
> > ===
> > Kernel Stack
> >
> > syscall_trace_enter+407
> > syscall_trace_enter+407
> > do_syscall_64+74
> > entry_SYSCALL_64_after_hwframe+75
> > ===
> >
> > Reported-by: Salvatore Benedetto <salvabenedetto@xxxxxxxx>
> > Suggested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> > ---
> > kernel/trace/trace_syscalls.c | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
>
> Note, we need to solve the same for perf_call_bpf_exit().
>
> pw-bot: cr
>
BTW, we lived with this bug for years, so I suggest basing your fix on
top of bpf-next/master, no bpf/master, which will give people a bit of
time to validate that the fix works as expected and doesn't produce
any undesirable side effects, before this makes it into the final
Linux release.
> > diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
> > index 9c581d6da843..063f51952d49 100644
> > --- a/kernel/trace/trace_syscalls.c
> > +++ b/kernel/trace/trace_syscalls.c
> > @@ -559,12 +559,15 @@ static int perf_call_bpf_enter(struct trace_event_call *call, struct pt_regs *re
>
> let's also drop struct pt_regs * argument into
> perf_call_bpf_{enter,exit}(), they are not actually used anymore
>
> > int syscall_nr;
> > unsigned long args[SYSCALL_DEFINE_MAXARGS];
> > } __aligned(8) param;
> > + struct pt_regs fake_regs;
> > int i;
> >
> > BUILD_BUG_ON(sizeof(param.ent) < sizeof(void *));
> >
> > /* bpf prog requires 'regs' to be the first member in the ctx (a.k.a. ¶m) */
> > - *(struct pt_regs **)¶m = regs;
> > + memset(&fake_regs, 0, sizeof(fake_regs));
>
> sizeof(struct pt_regs) == 168 on x86-64, and on arm64 it's a whopping
> 336 bytes, so these memset(0) calls are not free for sure.
>
> But we don't need to do this unnecessary work all the time.
>
> I initially was going to suggest to use get_bpf_raw_tp_regs() from
> kernel/trace/bpf_trace.c to get a temporary pt_regs that was already
> memset(0) and used to initialize these minimal "fake regs".
>
> But, it turns out we don't need to do even that. Note
> perf_trace_buf_alloc(), it has `struct pt_regs **` second argument,
> and if you pass a valid pointer there, it will return "fake regs"
> struct to be used. We already use that functionality in
> perf_trace_##call in include/trace/perf.h (i.e., non-syscall
> tracepoints), so this seems to be a perfect fit.
>
> > + perf_fetch_caller_regs(&fake_regs);
> > + *(struct pt_regs **)¶m = &fake_regs;
> > param.syscall_nr = rec->nr;
> > for (i = 0; i < sys_data->nb_args; i++)
> > param.args[i] = rec->args[i];
> > --
> > 2.43.5
> >
