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Date: Fri, 26 Jun 2020 23:47:37 +0000 From: Song Liu <songliubraving@...com> To: Andrii Nakryiko <andrii.nakryiko@...il.com> CC: bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>, Peter Ziljstra <peterz@...radead.org>, "Alexei Starovoitov" <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, "Kernel Team" <Kernel-team@...com>, john fastabend <john.fastabend@...il.com>, "KP Singh" <kpsingh@...omium.org> Subject: Re: [PATCH v2 bpf-next 2/4] bpf: introduce helper bpf_get_task_stak() > On Jun 26, 2020, at 3:51 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote: > > On Fri, Jun 26, 2020 at 3:45 PM Song Liu <songliubraving@...com> wrote: >> >> >> >>> On Jun 26, 2020, at 1:17 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote: >>> >>> On Thu, Jun 25, 2020 at 5:14 PM Song Liu <songliubraving@...com> wrote: >>>> >>>> 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 <songliubraving@...com> >>>> --- >>> >>> 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 <andriin@...com> >> >> Thanks! >> >>> >>>> 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(-) >>>> >>> >>> [...] >>> >>>> + /* 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? The buffer is sized for sysctl_perf_event_max_stack u64 numbers. stack_trace_save_tsk() will put at most stack_trace_save_tsk unsigned long in it (init_nr == 0). So the buffer is big enough. Thanks, Song
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