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bpf: Relax tracing prog recursive attach rules

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Currently, it's not allowed to attach an fentry/fexit prog to another
one fentry/fexit. At the same time it's not uncommon to see a tracing
program with lots of logic in use, and the attachment limitation
prevents usage of fentry/fexit for performance analysis (e.g. with
"bpftool prog profile" command) in this case. An example could be
falcosecurity libs project that uses tp_btf tracing programs.

Following the corresponding discussion [1], the reason for that is to
avoid tracing progs call cycles without introducing more complex
solutions. But currently it seems impossible to load and attach tracing
programs in a way that will form such a cycle. The limitation is coming
from the fact that attach_prog_fd is specified at the prog load (thus
making it impossible to attach to a program loaded after it in this
way), as well as tracing progs not implementing link_detach.

Replace "no same type" requirement with verification that no more than
one level of attachment nesting is allowed. In this way only one
fentry/fexit program could be attached to another fentry/fexit to cover
profiling use case, and still no cycle could be formed. To implement,
add a new field into bpf_prog_aux to track nested attachment for tracing
programs.

[1]: https://lore.kernel.org/bpf/20191108064039.2041889-16-ast@kernel.org/

Acked-by: Jiri Olsa <olsajiri@gmail.com>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
Link: https://lore.kernel.org/r/20240103190559.14750-2-9erthalion6@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Dmitrii Dolgov 2024-01-03 20:05:44 +01:00 committed by Alexei Starovoitov
parent 00bc898880
commit 19bfcdf949
3 changed files with 48 additions and 15 deletions
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1
include/linux/bpf.h
View file

@ -1449,6 +1449,7 @@ struct bpf_prog_aux {
bool dev_bound; /* Program is bound to the netdev. */
bool offload_requested; /* Program is bound and offloaded to the netdev. */
bool attach_btf_trace; /* true if attaching to BTF-enabled raw tp */
bool attach_tracing_prog; /* true if tracing another tracing program */
bool func_proto_unreliable;
bool sleepable;
bool tail_call_reachable;

23
kernel/bpf/syscall.c
View file

@ -2738,6 +2738,22 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
goto free_prog_sec;
}
/*
* Bookkeeping for managing the program attachment chain.
*
* It might be tempting to set attach_tracing_prog flag at the attachment
* time, but this will not prevent from loading bunch of tracing prog
* first, then attach them one to another.
*
* The flag attach_tracing_prog is set for the whole program lifecycle, and
* doesn't have to be cleared in bpf_tracing_link_release, since tracing
* programs cannot change attachment target.
*/
if (type == BPF_PROG_TYPE_TRACING && dst_prog &&
dst_prog->type == BPF_PROG_TYPE_TRACING) {
prog->aux->attach_tracing_prog = true;
}
/* find program type: socket_filter vs tracing_filter */
err = find_prog_type(type, prog);
if (err < 0)
@ -3171,7 +3187,12 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
}
if (tgt_prog_fd) {
/* For now we only allow new targets for BPF_PROG_TYPE_EXT */
/*
* For now we only allow new targets for BPF_PROG_TYPE_EXT. If this
* part would be changed to implement the same for
* BPF_PROG_TYPE_TRACING, do not forget to update the way how
* attach_tracing_prog flag is set.
*/
if (prog->type != BPF_PROG_TYPE_EXT) {
err = -EINVAL;
goto out_put_prog;

39
kernel/bpf/verifier.c
View file

@ -20317,6 +20317,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
struct bpf_attach_target_info *tgt_info)
{
bool prog_extension = prog->type == BPF_PROG_TYPE_EXT;
bool prog_tracing = prog->type == BPF_PROG_TYPE_TRACING;
const char prefix[] = "btf_trace_";
int ret = 0, subprog = -1, i;
const struct btf_type *t;
@ -20387,10 +20388,21 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
bpf_log(log, "Can attach to only JITed progs\n");
return -EINVAL;
}
if (tgt_prog->type == prog->type) {
/* Cannot fentry/fexit another fentry/fexit program.
* Cannot attach program extension to another extension.
* It's ok to attach fentry/fexit to extension program.
if (prog_tracing) {
if (aux->attach_tracing_prog) {
/*
* Target program is an fentry/fexit which is already attached
* to another tracing program. More levels of nesting
* attachment are not allowed.
*/
bpf_log(log, "Cannot nest tracing program attach more than once\n");
return -EINVAL;
}
} else if (tgt_prog->type == prog->type) {
/*
* To avoid potential call chain cycles, prevent attaching of a
* program extension to another extension. It's ok to attach
* fentry/fexit to extension program.
*/
bpf_log(log, "Cannot recursively attach\n");
return -EINVAL;
@ -20403,16 +20415,15 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
* except fentry/fexit. The reason is the following.
* The fentry/fexit programs are used for performance
* analysis, stats and can be attached to any program
* type except themselves. When extension program is
* replacing XDP function it is necessary to allow
* performance analysis of all functions. Both original
* XDP program and its program extension. Hence
* attaching fentry/fexit to BPF_PROG_TYPE_EXT is
* allowed. If extending of fentry/fexit was allowed it
* would be possible to create long call chain
* fentry->extension->fentry->extension beyond
* reasonable stack size. Hence extending fentry is not
* allowed.
* type. When extension program is replacing XDP function
* it is necessary to allow performance analysis of all
* functions. Both original XDP program and its program
* extension. Hence attaching fentry/fexit to
* BPF_PROG_TYPE_EXT is allowed. If extending of
* fentry/fexit was allowed it would be possible to create
* long call chain fentry->extension->fentry->extension
* beyond reasonable stack size. Hence extending fentry
* is not allowed.
*/
bpf_log(log, "Cannot extend fentry/fexit\n");
return -EINVAL;