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Message-ID: <660fd781-2d86-9d99-2851-127d6b4d4595@iogearbox.net>
Date: Wed, 7 Dec 2022 10:09:07 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Xin Liu <liuxin350@...wei.com>, andrii@...nel.org, ast@...nel.org,
martin.lau@...ux.dev, song@...nel.org, yhs@...com,
john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org,
linux-kernel@...r.kernel.org, yanan@...wei.com,
wuchangye@...wei.com, xiesongyang@...wei.com,
kongweibin2@...wei.com, zhangmingyi5@...wei.com
Subject: Re: [PATCH bpf-next] libbpf: Optimized return value in
libbpf_strerror when errno is libbpf errno
On 12/7/22 1:00 AM, Andrii Nakryiko wrote:
> On Mon, Dec 5, 2022 at 1:11 PM Daniel Borkmann <daniel@...earbox.net> 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@...wei.com>
>>> ---
>>> 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:
# 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)
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