Message-ID: <CAEf4BzYUMmpg2hqvaGphDzKSGKcS3zyTgxsyh_8iP79==jpVnQ@mail.gmail.com>
Date: Tue, 6 May 2025 14:38:41 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Lorenz Bauer <lmb@...valent.com>
Cc: Arnd Bergmann <arnd@...db.de>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 3/3] libbpf: Use mmap to parse vmlinux BTF
 from sysfs

On Mon, May 5, 2025 at 11:39 AM Lorenz Bauer <lmb@...valent.com> wrote:
>
> Teach libbpf to use mmap when parsing vmlinux BTF from /sys. We don't
> apply this to fall-back paths on the regular file system because there
> is no way to ensure that modifications underlying the MAP_PRIVATE
> mapping are not visible to the process.
>
> Signed-off-by: Lorenz Bauer <lmb@...valent.com>
> ---
>  tools/lib/bpf/btf.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 72 insertions(+), 11 deletions(-)
>

[...]

> @@ -1030,7 +1045,7 @@ struct btf *btf__new_empty_split(struct btf *base_btf)
>         return libbpf_ptr(btf_new_empty(base_btf));
>  }
>
> -static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
> +static struct btf *btf_new_no_copy(void *data, __u32 size, struct btf *base_btf)
>  {
>         struct btf *btf;
>         int err;
> @@ -1050,12 +1065,7 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
>                 btf->start_str_off = base_btf->hdr->str_len;
>         }
>
> -       btf->raw_data = malloc(size);
> -       if (!btf->raw_data) {
> -               err = -ENOMEM;
> -               goto done;
> -       }
> -       memcpy(btf->raw_data, data, size);
> +       btf->raw_data = data;
>         btf->raw_size = size;
>
>         btf->hdr = btf->raw_data;
> @@ -1081,6 +1091,24 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
>         return btf;
>  }
>
> +static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)

btf_new() is internal, so I'd extend existing btf_new() with `bool
is_mmap` and not add btf_new_no_copy(), I think it's simpler.
Eventually we can turn is_mmap into some sort of flags, if we need
more tuning of data ownership behavior

> +{
> +       struct btf *btf;
> +       void *raw_data;
> +
> +       raw_data = malloc(size);
> +       if (!raw_data)
> +               return ERR_PTR(-ENOMEM);
> +
> +       memcpy(raw_data, data, size);
> +
> +       btf = btf_new_no_copy(raw_data, size, base_btf);
> +       if (IS_ERR(btf))
> +               free(raw_data);
> +
> +       return btf;
> +}
> +
>  struct btf *btf__new(const void *data, __u32 size)
>  {
>         return libbpf_ptr(btf_new(data, size, NULL));
> @@ -1354,6 +1382,37 @@ struct btf *btf__parse_raw_split(const char *path, struct btf *base_btf)
>         return libbpf_ptr(btf_parse_raw(path, base_btf));
>  }
>
> +static struct btf *btf_parse_raw_mmap(const char *path, struct btf *base_btf)
> +{
> +       struct stat st;
> +       void *data;
> +       struct btf *btf;
> +       int fd;
> +
> +       fd = open(path, O_RDONLY);
> +       if (fd < 0)
> +               return libbpf_err_ptr(-errno);
> +
> +       if (fstat(fd, &st) < 0) {
> +               close(fd);

close() can clobber errno, so save `err = -errno` before it

> +               return libbpf_err_ptr(-errno);
> +       }
> +
> +       data = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
> +       close(fd);

same, errno clobbering danger

> +
> +       if (data == MAP_FAILED)
> +               return NULL;
> +
> +       btf = btf_new_no_copy(data, st.st_size, base_btf);
> +       if (!btf)

btf_new_no_copy() is returning ERR_PTR() on error, no?

pw-bot: cr

> +               munmap(data, st.st_size);
> +       else
> +               btf->raw_data_is_mmap = true;
> +
> +       return btf;
> +}
> +
>  static struct btf *btf_parse(const char *path, struct btf *base_btf, struct btf_ext **btf_ext)
>  {
>         struct btf *btf;
> @@ -1659,8 +1718,7 @@ struct btf *btf__load_from_kernel_by_id(__u32 id)
>  static void btf_invalidate_raw_data(struct btf *btf)
>  {
>         if (btf->raw_data) {
> -               free(btf->raw_data);
> -               btf->raw_data = NULL;
> +               btf_free_raw_data(btf);
>         }
>         if (btf->raw_data_swapped) {
>                 free(btf->raw_data_swapped);
> @@ -5290,7 +5348,10 @@ struct btf *btf__load_vmlinux_btf(void)
>                 pr_warn("kernel BTF is missing at '%s', was CONFIG_DEBUG_INFO_BTF enabled?\n",
>                         sysfs_btf_path);
>         } else {
> -               btf = btf__parse(sysfs_btf_path, NULL);
> +               btf = btf_parse_raw_mmap(sysfs_btf_path, NULL);
> +               if (IS_ERR_OR_NULL(btf))
> +                       btf = btf__parse(sysfs_btf_path, NULL);
> +
>                 if (!btf) {
>                         err = -errno;
>                         pr_warn("failed to read kernel BTF from '%s': %s\n",
>
> --
> 2.49.0
>

Powered by blists - more mailing lists