Re: [PATCH bpf-next] libbpf: Optimized return value in libbpf_strerror when errno is libbpf errno

[Andrii Nakryiko]

On Wed, Dec 7, 2022 at 1:09 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 12/7/22 1:00 AM, Andrii Nakryiko wrote:
> > On Mon, Dec 5, 2022 at 1:11 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> >>
> >> On 12/3/22 10:37 AM, Xin Liu wrote:
> >>> This is a small improvement in libbpf_strerror. When libbpf_strerror
> >>> is used to obtain the system error description, if the length of the
> >>> buf is insufficient, libbpf_sterror returns ERANGE and sets errno to
> >>> ERANGE.
> >>>
> >>> However, this processing is not performed when the error code
> >>> customized by libbpf is obtained. Make some minor improvements here,
> >>> return -ERANGE and set errno to ERANGE when buf is not enough for
> >>> custom description.
> >>>
> >>> Signed-off-by: Xin Liu <liuxin350@xxxxxxxxxx>
> >>> ---
> >>>    tools/lib/bpf/libbpf_errno.c | 6 ++++++
> >>>    1 file changed, 6 insertions(+)
> >>>
> >>> diff --git a/tools/lib/bpf/libbpf_errno.c b/tools/lib/bpf/libbpf_errno.c
> >>> index 96f67a772a1b..48ce7d5b5bf9 100644
> >>> --- a/tools/lib/bpf/libbpf_errno.c
> >>> +++ b/tools/lib/bpf/libbpf_errno.c
> >>> @@ -54,10 +54,16 @@ int libbpf_strerror(int err, char *buf, size_t size)
> >>>
> >>>        if (err < __LIBBPF_ERRNO__END) {
> >>>                const char *msg;
> >>> +             size_t msg_size;
> >>>
> >>>                msg = libbpf_strerror_table[ERRNO_OFFSET(err)];
> >>>                snprintf(buf, size, "%s", msg);
> >>>                buf[size - 1] = '\0';
> >>> +
> >>> +             msg_size = strlen(msg);
> >>> +             if (msg_size >= size)
> >>> +                     return libbpf_err(-ERANGE);
> >>
> >> Given this is related to libbpf_strerror_table[] where the error strings are known
> >> lets do compile-time error instead. All callers should pass in a buffer of STRERR_BUFSIZE
> >> size in libbpf.
> >
> > That sounds a bit too pessimistic?.. If the actual error message fits
> > in the buffer, why return -ERANGE just because theoretically some
> > error descriptions might fit?
> >
> > But I don't think we need to calculate strlen(). snprintf above
> > returns the number of bytes required to print a full string, even if
> > it was truncated. So just comparing snprintf's result to size should
> > be enough.
>
> I meant sth like below. For example if we were to shrink STRERR_BUFSIZE down
> to 32 for testing, you'd then get:

Sure, but I'm not sure why do we need to do this? Array of pointers to
string will overall use less memory, as each string will take as much
space as needed and no more.

I guess I'm missing which problem you are trying to solve. I believe
Xin was addressing a concern of extern (to libbpf) callers not getting
-ERANGE in cases when they provide too small a buffer, which is just a
simple snprintf() use adjustment to be a proper fix.

>
> # make libbpf_errno.o
> gcc -g -O2 -std=gnu89 -Wbad-function-cast -Wdeclaration-after-statement -Wformat-security -Wformat-y2k -Winit-self -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wno-system-headers -Wold-style-definition -Wpacked -Wredundant-decls -Wstrict-prototypes -Wswitch-default -Wswitch-enum -Wundef -Wwrite-strings -Wformat -Wno-type-limits -Wstrict-aliasing=3 -Wshadow -Wno-switch-enum -Werror -Wall -I. -I/home/darkstar/trees/bpf-next/tools/include -I/home/darkstar/trees/bpf-next/tools/include/uapi -fvisibility=hidden -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64    -c -o libbpf_errno.o libbpf_errno.c
> libbpf_errno.c:27:31: error: initializer-string for array of chars is too long [-Werror]
>     27 |  [ERRCODE_OFFSET(KVERSION)] = "'version' section incorrect or lost",
>        |                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> libbpf_errno.c:27:31: note: (near initialization for ‘libbpf_strerror_table[2]’)
> libbpf_errno.c:31:29: error: initializer-string for array of chars is too long [-Werror]
>     31 |  [ERRCODE_OFFSET(VERIFY)] = "Kernel verifier blocks program loading",
>        |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> libbpf_errno.c:31:29: note: (near initialization for ‘libbpf_strerror_table[7]’)
> libbpf_errno.c:34:31: error: initializer-string for array of chars is too long [-Werror]
>     34 |  [ERRCODE_OFFSET(PROGTYPE)] = "Kernel doesn't support this program type",
>        |                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> libbpf_errno.c:34:31: note: (near initialization for ‘libbpf_strerror_table[10]’)
> libbpf_errno.c:37:30: error: initializer-string for array of chars is too long [-Werror]
>     37 |  [ERRCODE_OFFSET(NLPARSE)] = "Incorrect netlink message parsing",
>        |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> libbpf_errno.c:37:30: note: (near initialization for ‘libbpf_strerror_table[13]’)
> cc1: all warnings being treated as errors
> make: *** [<builtin>: libbpf_errno.o] Error 1
>
>
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 2a82f49ce16f..2e5df1624f79 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -265,8 +265,6 @@ static void pr_perm_msg(int err)
>                 buf);
>   }
>
> -#define STRERR_BUFSIZE  128
> -
>   /* Copied from tools/perf/util/util.h */
>   #ifndef zfree
>   # define zfree(ptr) ({ free(*ptr); *ptr = NULL; })
> diff --git a/tools/lib/bpf/libbpf_errno.c b/tools/lib/bpf/libbpf_errno.c
> index 96f67a772a1b..2f03f861b8b6 100644
> --- a/tools/lib/bpf/libbpf_errno.c
> +++ b/tools/lib/bpf/libbpf_errno.c
> @@ -21,7 +21,7 @@
>   #define ERRCODE_OFFSET(c)     ERRNO_OFFSET(LIBBPF_ERRNO__##c)
>   #define NR_ERRNO      (__LIBBPF_ERRNO__END - __LIBBPF_ERRNO__START)
>
> -static const char *libbpf_strerror_table[NR_ERRNO] = {
> +static const char libbpf_strerror_table[NR_ERRNO][STRERR_BUFSIZE] = {
>         [ERRCODE_OFFSET(LIBELF)]        = "Something wrong in libelf",
>         [ERRCODE_OFFSET(FORMAT)]        = "BPF object format invalid",
>         [ERRCODE_OFFSET(KVERSION)]      = "'version' section incorrect or lost",
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 377642ff51fc..d4dc4fe945a6 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -57,6 +57,8 @@
>   #define ELF64_ST_VISIBILITY(o) ((o) & 0x03)
>   #endif
>
> +#define STRERR_BUFSIZE 128
> +
>   #define BTF_INFO_ENC(kind, kind_flag, vlen) \
>         ((!!(kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))
>   #define BTF_TYPE_ENC(name, info, size_or_type) (name), (info), (size_or_type)