Re: [PATCH v2 bpf-next 2/4] bpf: introduce helper bpf_get_task_stak()

On Fri, Jun 26, 2020 at 3:45 PM Song Liu [off-list ref] wrote:

quoted

On Jun 26, 2020, at 1:17 PM, Andrii Nakryiko [off-list ref] wrote:

On Thu, Jun 25, 2020 at 5:14 PM Song Liu [off-list ref] wrote:

quoted

Introduce helper bpf_get_task_stack(), which dumps stack trace of given
task. This is different to bpf_get_stack(), which gets stack track of
current task. One potential use case of bpf_get_task_stack() is to call
it from bpf_iter__task and dump all /proc/<pid>/stack to a seq_file.

bpf_get_task_stack() uses stack_trace_save_tsk() instead of
get_perf_callchain() for kernel stack. The benefit of this choice is that
stack_trace_save_tsk() doesn't require changes in arch/. The downside of
using stack_trace_save_tsk() is that stack_trace_save_tsk() dumps the
stack trace to unsigned long array. For 32-bit systems, we need to
translate it to u64 array.

Signed-off-by: Song Liu <redacted>
---

Looks great, I just think that there are cases where user doesn't
necessarily has valid task_struct pointer, just pid, so would be nice
to not artificially restrict such cases by having extra helper.

Acked-by: Andrii Nakryiko <redacted>

Thanks!

quoted

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include/linux/bpf.h            |  1 +
include/uapi/linux/bpf.h       | 35 ++++++++++++++-
kernel/bpf/stackmap.c          | 79 ++++++++++++++++++++++++++++++++--
kernel/trace/bpf_trace.c       |  2 +
scripts/bpf_helpers_doc.py     |  2 +
tools/include/uapi/linux/bpf.h | 35 ++++++++++++++-
6 files changed, 149 insertions(+), 5 deletions(-)

[...]

quoted

+       /* stack_trace_save_tsk() works on unsigned long array, while
+        * perf_callchain_entry uses u64 array. For 32-bit systems, it is
+        * necessary to fix this mismatch.
+        */
+       if (__BITS_PER_LONG != 64) {
+               unsigned long *from = (unsigned long *) entry->ip;
+               u64 *to = entry->ip;
+               int i;
+
+               /* copy data from the end to avoid using extra buffer */
+               for (i = entry->nr - 1; i >= (int)init_nr; i--)
+                       to[i] = (u64)(from[i]);

doing this forward would be just fine as well, no? First iteration
will cast and overwrite low 32-bits, all the subsequent iterations
won't even overlap.

I think first iteration will write zeros to higher 32 bits, no?

Oh, wait, I completely misread what this is doing. It up-converts from
32-bit to 64-bit, sorry. Yeah, ignore me on this :)

But then I have another question. How do you know that entry->ip has
enough space to keep the same number of 2x bigger entries?

quoted

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+       }
+
+exit_put:
+       put_callchain_entry(rctx);
+
+       return entry;
+}
+

[...]

quoted

+BPF_CALL_4(bpf_get_task_stack, struct task_struct *, task, void *, buf,
+          u32, size, u64, flags)
+{
+       struct pt_regs *regs = task_pt_regs(task);
+
+       return __bpf_get_stack(regs, task, buf, size, flags);
+}

So this takes advantage of BTF and having a direct task_struct
pointer. But for kprobes/tracepoint I think it would also be extremely
helpful to be able to request stack trace by PID. How about one more
helper which will wrap this one with get/put task by PID, e.g.,
bpf_get_pid_stack(int pid, void *buf, u32 size, u64 flags)? Would that
be a problem?

That should work. Let me add that in a follow up patch.