Date:   Thu, 9 Feb 2023 09:16:46 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <olsajiri@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Hao Luo <haoluo@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        bpf@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-perf-users@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH RFC 5/5] selftests/bpf: Add iter_task_vma_buildid test

On Thu, Feb 9, 2023 at 6:04 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Wed, Feb 08, 2023 at 04:01:42PM -0800, Andrii Nakryiko wrote:
>
> SNIP
>
> > > +static void test_task_vma_buildid(void)
> > > +{
> > > +       int err, iter_fd = -1, proc_maps_fd = -1;
> > > +       struct bpf_iter_task_vma_buildid *skel;
> > > +       char key[D_PATH_BUF_SIZE], *prev_key;
> > > +       char bpf_build_id[BUILDID_STR_SIZE];
> > > +       int len, files_fd, i, cnt = 0;
> > > +       struct build_id val;
> > > +       char *build_id;
> > > +       char c;
> > > +
> > > +       skel = bpf_iter_task_vma_buildid__open();
> > > +       if (!ASSERT_OK_PTR(skel, "bpf_iter_task_vma_buildid__open"))
> > > +               return;
> > > +
> > > +       err = bpf_iter_task_vma_buildid__load(skel);
> > > +       if (!ASSERT_OK(err, "bpf_iter_task_vma_buildid__load"))
> > > +               goto out;
> >
> > minor: you can do __open_and_load() in one step
>
> right, I copied that from another test, but removed all the
> setup in between, so we can actually call just __open_and_load
>
> SNIP
>
> > > +               memset(bpf_build_id, 0x0, sizeof(bpf_build_id));
> > > +               for (i = 0; i < val.sz; i++) {
> > > +                       sprintf(bpf_build_id + i*2, "%02x",
> > > +                               (unsigned char) val.data[i]);
> > > +               }
> > > +
> > > +               if (!ASSERT_OK(read_buildid(key, &build_id), "read_buildid"))
> > > +                       break;
> > > +
> > > +               printf("BUILDID %s %s %s\n", bpf_build_id, build_id, key);
> >
> > debugging leftover or intentional?
> >
> > > +               ASSERT_OK(strncmp(bpf_build_id, build_id, strlen(bpf_build_id)), "buildid_cmp");
> > > +
> > > +               free(build_id);
> > > +               prev_key = key;
> > > +               cnt++;
> > > +       }
> > > +
> > > +       printf("checked %d files\n", cnt);
> >
> > ditto
>
> both intentional, first one can go out I guess, but the
> number of checked files seemed interesting to me ;-)
>
> SNIP
>
> > > diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_task_vma_buildid.c b/tools/testing/selftests/bpf/progs/bpf_iter_task_vma_buildid.c
> > > new file mode 100644
> > > index 000000000000..25e2179ae5f4
> > > --- /dev/null
> > > +++ b/tools/testing/selftests/bpf/progs/bpf_iter_task_vma_buildid.c
> > > @@ -0,0 +1,49 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +
> > > +#include "bpf_iter.h"
> > > +#include <bpf/bpf_helpers.h>
> > > +#include <string.h>
> > > +
> > > +char _license[] SEC("license") = "GPL";
> > > +
> > > +#define VM_EXEC                0x00000004
> > > +#define D_PATH_BUF_SIZE        1024
> > > +
> > > +struct {
> > > +       __uint(type, BPF_MAP_TYPE_HASH);
> > > +       __uint(max_entries, 10000);
> > > +       __type(key, char[D_PATH_BUF_SIZE]);
> > > +       __type(value, struct build_id);
> > > +} files SEC(".maps");
> > > +
> > > +static char tmp_key[D_PATH_BUF_SIZE];
> > > +static struct build_id tmp_data;
> > > +
> > > +SEC("iter/task_vma") int proc_maps(struct bpf_iter__task_vma *ctx)
> >
> > nit: let's keep SEC() on separate line from function itself
>
> ok
>
> >
> > > +{
> > > +       struct vm_area_struct *vma = ctx->vma;
> > > +       struct seq_file *seq = ctx->meta->seq;
> > > +       struct task_struct *task = ctx->task;
> > > +       unsigned long file_key;
> > > +       struct file *file;
> > > +
> > > +       if (task == (void *)0 || vma == (void *)0)
> > > +               return 0;
> > > +
> > > +       if (!(vma->vm_flags & VM_EXEC))
> > > +               return 0;
> > > +
> > > +       file = vma->vm_file;
> > > +       if (!file)
> > > +               return 0;
> > > +
> > > +       memset(tmp_key, 0x0, D_PATH_BUF_SIZE);
> >
> > __builtin_memset() to not rely on compiler optimization?
> >
> > > +       bpf_d_path(&file->f_path, (char *) &tmp_key, D_PATH_BUF_SIZE);
> > > +
> > > +       if (bpf_map_lookup_elem(&files, &tmp_key))
> > > +               return 0;
> > > +
> > > +       memcpy(&tmp_data, file->f_bid, sizeof(*file->f_bid));
> >
> > same about __builtin_memcpy()
>
> ah ok, did not know that, will check.. curious what could
> go wrong by using not '__builtin_...' version?

if compiler doesn't optimize it into __builtin_memcpy() (which results
in just explicit assembly code to copy/set data word-by-word), then
BPF program will do actual call to memset(), which with C rules would
be inferred as extern symbol, which would fail BPF object loading with
error along the lines of "couldn't resolve memset extern".

>
> thanks,
> jirka

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