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bpf: Add bpf_task_pt_regs() helper

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The motivation behind this helper is to access userspace pt_regs in a
kprobe handler.

uprobe's ctx is the userspace pt_regs. kprobe's ctx is the kernelspace
pt_regs. bpf_task_pt_regs() allows accessing userspace pt_regs in a
kprobe handler. The final case (kernelspace pt_regs in uprobe) is
pretty rare (usermode helper) so I think that can be solved later if
necessary.

More concretely, this helper is useful in doing BPF-based DWARF stack
unwinding. Currently the kernel can only do framepointer based stack
unwinds for userspace code. This is because the DWARF state machines are
too fragile to be computed in kernelspace [0]. The idea behind
DWARF-based stack unwinds w/ BPF is to copy a chunk of the userspace
stack (while in prog context) and send it up to userspace for unwinding
(probably with libunwind) [1]. This would effectively enable profiling
applications with -fomit-frame-pointer using kprobes and uprobes.

[0]: https://lkml.org/lkml/2012/2/10/356
[1]: https://github.com/danobi/bpf-dwarf-walk

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/e2718ced2d51ef4268590ab8562962438ab82815.1629772842.git.dxu@dxuuu.xyz
This commit is contained in:
Daniel Xu 2021-08-23 19:43:49 -07:00 • committed by Alexei Starovoitov
parent a396eda551
commit dd6e10fbd9
4 changed files with 36 additions and 0 deletions
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19
kernel/trace/bpf_trace.c
View file

@ -723,6 +723,23 @@ const struct bpf_func_proto bpf_get_current_task_btf_proto = {
.ret_btf_id = &btf_task_struct_ids[0],
};
BPF_CALL_1(bpf_task_pt_regs, struct task_struct *, task)
{
return (unsigned long) task_pt_regs(task);
}
BTF_ID_LIST(bpf_task_pt_regs_ids)
BTF_ID(struct, pt_regs)
const struct bpf_func_proto bpf_task_pt_regs_proto = {
.func = bpf_task_pt_regs,
.gpl_only = true,
.arg1_type = ARG_PTR_TO_BTF_ID,
.arg1_btf_id = &btf_task_struct_ids[0],
.ret_type = RET_PTR_TO_BTF_ID,
.ret_btf_id = &bpf_task_pt_regs_ids[0],
};
BPF_CALL_2(bpf_current_task_under_cgroup, struct bpf_map *, map, u32, idx)
{
struct bpf_array *array = container_of(map, struct bpf_array, map);
@ -1032,6 +1049,8 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
return &bpf_get_current_task_proto;
case BPF_FUNC_get_current_task_btf:
return &bpf_get_current_task_btf_proto;
case BPF_FUNC_task_pt_regs:
return &bpf_task_pt_regs_proto;
case BPF_FUNC_get_current_uid_gid:
return &bpf_get_current_uid_gid_proto;
case BPF_FUNC_get_current_comm: