Hello Hyeonggon,
On Sun, Sep 29, 2024 at 11:27:25PM +0900, Hyeonggon Yoo wrote:
> On Sun, Sep 29, 2024 at 3:13 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >
> > On Fri, Sep 27, 2024 at 11:41:33AM -0700, Namhyung Kim wrote:
> > > The test traverses all slab caches using the kmem_cache_iter and check
> > > if current task's pointer is from "task_struct" slab cache.
> > >
> > > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > > ---
> > > .../bpf/prog_tests/kmem_cache_iter.c | 64 ++++++++++++++++++
> > > tools/testing/selftests/bpf/progs/bpf_iter.h | 7 ++
> > > .../selftests/bpf/progs/kmem_cache_iter.c | 66 +++++++++++++++++++
> > > 3 files changed, 137 insertions(+)
> > > create mode 100644 tools/testing/selftests/bpf/prog_tests/kmem_cache_iter.c
> > > create mode 100644 tools/testing/selftests/bpf/progs/kmem_cache_iter.c
> > >
> > > diff --git a/tools/testing/selftests/bpf/prog_tests/kmem_cache_iter.c b/tools/testing/selftests/bpf/prog_tests/kmem_cache_iter.c
> > > new file mode 100644
> > > index 0000000000000000..814bcc453e9f3ccd
> > > --- /dev/null
> > > +++ b/tools/testing/selftests/bpf/prog_tests/kmem_cache_iter.c
> > > @@ -0,0 +1,64 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/* Copyright (c) 2024 Google */
> > > +
> > > +#include <test_progs.h>
> > > +#include <bpf/libbpf.h>
> > > +#include <bpf/btf.h>
> > > +#include "kmem_cache_iter.skel.h"
> > > +
> > > +static void test_kmem_cache_iter_check_task(struct kmem_cache_iter *skel)
> > > +{
> > > + LIBBPF_OPTS(bpf_test_run_opts, opts,
> > > + .flags = BPF_F_TEST_RUN_ON_CPU,
> > > + );
> > > + int prog_fd = bpf_program__fd(skel->progs.check_task_struct);
> > > +
> > > + /* get task_struct and check it if's from a slab cache */
> > > + bpf_prog_test_run_opts(prog_fd, &opts);
> > > +
> > > + /* the BPF program should set 'found' variable */
> > > + ASSERT_EQ(skel->bss->found, 1, "found task_struct");
> >
> > Hmm.. I'm seeing a failure with found being -1, which means ...
> >
> > > +}
> > > +
> > > +void test_kmem_cache_iter(void)
> > > +{
> > > + DECLARE_LIBBPF_OPTS(bpf_iter_attach_opts, opts);
> > > + struct kmem_cache_iter *skel = NULL;
> > > + union bpf_iter_link_info linfo = {};
> > > + struct bpf_link *link;
> > > + char buf[1024];
> > > + int iter_fd;
> > > +
> > > + skel = kmem_cache_iter__open_and_load();
> > > + if (!ASSERT_OK_PTR(skel, "kmem_cache_iter__open_and_load"))
> > > + return;
> > > +
> > > + opts.link_info = &linfo;
> > > + opts.link_info_len = sizeof(linfo);
> > > +
> > > + link = bpf_program__attach_iter(skel->progs.slab_info_collector, &opts);
> > > + if (!ASSERT_OK_PTR(link, "attach_iter"))
> > > + goto destroy;
> > > +
> > > + iter_fd = bpf_iter_create(bpf_link__fd(link));
> > > + if (!ASSERT_GE(iter_fd, 0, "iter_create"))
> > > + goto free_link;
> > > +
> > > + memset(buf, 0, sizeof(buf));
> > > + while (read(iter_fd, buf, sizeof(buf) > 0)) {
> > > + /* read out all contents */
> > > + printf("%s", buf);
> > > + }
> > > +
> > > + /* next reads should return 0 */
> > > + ASSERT_EQ(read(iter_fd, buf, sizeof(buf)), 0, "read");
> > > +
> > > + test_kmem_cache_iter_check_task(skel);
> > > +
> > > + close(iter_fd);
> > > +
> > > +free_link:
> > > + bpf_link__destroy(link);
> > > +destroy:
> > > + kmem_cache_iter__destroy(skel);
> > > +}
> > > diff --git a/tools/testing/selftests/bpf/progs/bpf_iter.h b/tools/testing/selftests/bpf/progs/bpf_iter.h
> > > index c41ee80533ca219a..3305dc3a74b32481 100644
> > > --- a/tools/testing/selftests/bpf/progs/bpf_iter.h
> > > +++ b/tools/testing/selftests/bpf/progs/bpf_iter.h
> > > @@ -24,6 +24,7 @@
> > > #define BTF_F_PTR_RAW BTF_F_PTR_RAW___not_used
> > > #define BTF_F_ZERO BTF_F_ZERO___not_used
> > > #define bpf_iter__ksym bpf_iter__ksym___not_used
> > > +#define bpf_iter__kmem_cache bpf_iter__kmem_cache___not_used
> > > #include "vmlinux.h"
> > > #undef bpf_iter_meta
> > > #undef bpf_iter__bpf_map
> > > @@ -48,6 +49,7 @@
> > > #undef BTF_F_PTR_RAW
> > > #undef BTF_F_ZERO
> > > #undef bpf_iter__ksym
> > > +#undef bpf_iter__kmem_cache
> > >
> > > struct bpf_iter_meta {
> > > struct seq_file *seq;
> > > @@ -165,3 +167,8 @@ struct bpf_iter__ksym {
> > > struct bpf_iter_meta *meta;
> > > struct kallsym_iter *ksym;
> > > };
> > > +
> > > +struct bpf_iter__kmem_cache {
> > > + struct bpf_iter_meta *meta;
> > > + struct kmem_cache *s;
> > > +} __attribute__((preserve_access_index));
> > > diff --git a/tools/testing/selftests/bpf/progs/kmem_cache_iter.c b/tools/testing/selftests/bpf/progs/kmem_cache_iter.c
> > > new file mode 100644
> > > index 0000000000000000..3f6ec15a1bf6344c
> > > --- /dev/null
> > > +++ b/tools/testing/selftests/bpf/progs/kmem_cache_iter.c
> > > @@ -0,0 +1,66 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/* Copyright (c) 2024 Google */
> > > +
> > > +#include "bpf_iter.h"
> > > +#include <bpf/bpf_helpers.h>
> > > +#include <bpf/bpf_tracing.h>
> > > +
> > > +char _license[] SEC("license") = "GPL";
> > > +
> > > +#define SLAB_NAME_MAX 256
> > > +
> > > +struct {
> > > + __uint(type, BPF_MAP_TYPE_HASH);
> > > + __uint(key_size, sizeof(void *));
> > > + __uint(value_size, SLAB_NAME_MAX);
> > > + __uint(max_entries, 1024);
> > > +} slab_hash SEC(".maps");
> > > +
> > > +extern struct kmem_cache *bpf_get_kmem_cache(__u64 addr) __ksym;
> > > +
> > > +/* result, will be checked by userspace */
> > > +int found;
> > > +
> > > +SEC("iter/kmem_cache")
> > > +int slab_info_collector(struct bpf_iter__kmem_cache *ctx)
> > > +{
> > > + struct seq_file *seq = ctx->meta->seq;
> > > + struct kmem_cache *s = ctx->s;
> > > +
> > > + if (s) {
> > > + char name[SLAB_NAME_MAX];
> > > +
> > > + /*
> > > + * To make sure if the slab_iter implements the seq interface
> > > + * properly and it's also useful for debugging.
> > > + */
> > > + BPF_SEQ_PRINTF(seq, "%s: %u\n", s->name, s->object_size);
> > > +
> > > + bpf_probe_read_kernel_str(name, sizeof(name), s->name);
> > > + bpf_map_update_elem(&slab_hash, &s, name, BPF_NOEXIST);
> > > + }
> > > +
> > > + return 0;
> > > +}
> > > +
> > > +SEC("raw_tp/bpf_test_finish")
> > > +int BPF_PROG(check_task_struct)
> > > +{
> > > + __u64 curr = bpf_get_current_task();
> > > + struct kmem_cache *s;
> > > + char *name;
> > > +
> > > + s = bpf_get_kmem_cache(curr);
> > > + if (s == NULL) {
> > > + found = -1;
> > > + return 0;
> >
> > ... it cannot find a kmem_cache for the current task. This program is
> > run by bpf_prog_test_run_opts() with BPF_F_TEST_RUN_ON_CPU. So I think
> > the curr should point a task_struct in a slab cache.
> >
> > Am I missing something?
>
> Hi Namhyung,
>
> Out of curiosity I've been investigating this issue on my machine and
> running some experiments.
Thanks a lot for looking at this!
>
> When the test fails, calling dump_page() for the page the task_struct
> belongs to,
> shows that the page does not have the PGTY_slab flag set which is why
> virt_to_slab(current) returns NULL.
>
> Does the test always fails on your environment? On my machine, the
> test passed sometimes but failed some times.
I'm using vmtest.sh but it succeeded mostly. I thought I couldn't
reproduce it locally, but I also see the failure sometimes. I'll take a
deeper look.
>
> Maybe sometimes the value returned by 'current' macro belongs to a
> slab, but sometimes it does not.
> But that doesn't really make sense to me as IIUC task_struct
> descriptors are allocated from slab.
AFAIK the notable exception is the init_task which lives in the kernel
data. I'm not sure the if the test is running by PID 1.
>
> ....Or maybe some code can overwrote the page_type field of a slab?
> Hmm, it seems we need more information to identify what's gone wrong.
I doubt it's the case, but who knows? :)
>
> Just FYI, adding the output of the following code snippet in
> bpf_get_kmem_cache():
>
> pr_info("current = %llx\n", (unsigned long long)current);
> dump_page(virt_to_head_page(current), "virt_to_head_page()");
Thanks, I'll try this in my test too.
Namhyung
>
> # When the test passes
> [ 232.755028] current = ffff8ff5b9ebd200
> [ 232.755031] page: refcount:1 mapcount:0 mapping:0000000000000000
> index:0x0 pfn:0x139eb8
> [ 232.755033] head: order:3 mapcount:0 entire_mapcount:0
> nr_pages_mapped:0 pincount:0
> [ 232.755035] memcg:ffff8ff5b3ee0c01
> [ 232.755037] ksm flags:
> 0x17ffffc0000040(head|node=0|zone=2|lastcpupid=0x1fffff)
> [ 232.755040] page_type: f5(slab)
> [ 232.755042] raw: 0017ffffc0000040 ffff8ff58028ab00 ffffdaba05b8fc00
> dead000000000003
> [ 232.755045] raw: 0000000000000000 0000000000030003 00000001f5000000
> ffff8ff5b3ee0c01
> [ 232.755047] head: 0017ffffc0000040 ffff8ff58028ab00
> ffffdaba05b8fc00 dead000000000003
> [ 232.755048] head: 0000000000000000 0000000000030003
> 00000001f5000000 ffff8ff5b3ee0c01
> [ 232.755050] head: 0017ffffc0000003 ffffdaba04e7ae01
> ffffffffffffffff 0000000000000000
> [ 232.755052] head: 0000000000000008 0000000000000000
> 00000000ffffffff 0000000000000000
> [ 232.755053] page dumped because: virt_to_head_page()
>
> # When the test fails
> [ 130.811626] current = ffffffff884110c0
> [ 130.811628] page: refcount:1 mapcount:0 mapping:0000000000000000
> index:0x0 pfn:0x8a9411
> [ 130.811632] flags:
> 0x17ffffc0002000(reserved|node=0|zone=2|lastcpupid=0x1fffff)
> [ 130.811636] raw: 0017ffffc0002000 ffffdaba22a50448 ffffdaba22a50448
> 0000000000000000
> [ 130.811639] raw: 0000000000000000 0000000000000000 00000001ffffffff
> 0000000000000000
> [ 130.811641] page dumped because: virt_to_head_page()
>
> Best,
> Hyeonggon
>
> >
> > Thanks,
> > Namhyung
> >
> > > + }
> > > +
> > > + name = bpf_map_lookup_elem(&slab_hash, &s);
> > > + if (name && !bpf_strncmp(name, 11, "task_struct"))
> > > + found = 1;
> > > + else
> > > + found = -2;
> > > +
> > > + return 0;
> > > +}
> > > --
> > > 2.46.1.824.gd892dcdcdd-goog
> > >
