On Fri, Jul 7, 2023 at 9:08 PM Leon Hwang <hffilwlqm@xxxxxxxxx> wrote:
>
> Currently, excluding verifier, users are unable to obtain detailed error
> information when issues occur in BPF syscall.
>
> To overcome this limitation, bpf generic log is introduced to provide
> error details similar to the verifier. This enhancement will enable the
> reporting of error details along with the corresponding errno in BPF
> syscall.
>
> Essentially, bpf generic log functions as a mechanism similar to netlink,
> enabling the transmission of error messages to user space. This
> mechanism greatly enhances the usability of BPF syscall by allowing
> users to access comprehensive error messages instead of relying solely
> on errno.
>
> This patch specifically addresses the error reporting in dev_xdp_attach()
> . With this patch, the error messages will be transferred to user space
> like the netlink approach. Hence, users will be able to check the error
> message along with the errno.
>
> Signed-off-by: Leon Hwang <hffilwlqm@xxxxxxxxx>
> ---
> include/linux/bpf.h | 30 ++++++++++++++++++++++++++++++
> include/uapi/linux/bpf.h | 6 ++++++
> kernel/bpf/log.c | 33 +++++++++++++++++++++++++++++++++
> net/core/dev.c | 11 ++++++++++-
> tools/include/uapi/linux/bpf.h | 6 ++++++
> 5 files changed, 85 insertions(+), 1 deletion(-)
>
Just curious, what's wrong with struct bpf_verifier_log for
implementing "generic log"? bpf_log, bpf_vlog_reset, bpf_vlog_finalize
are quite generic, I think. Why invent yet another structure? Existing
code and struct support rotating log, if necessary.
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 360433f14496a..7d2124a537943 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -3107,4 +3107,34 @@ static inline gfp_t bpf_memcg_flags(gfp_t flags)
> return flags;
> }
>
> +#define BPF_GENERIC_TMP_LOG_SIZE 256
> +
> +struct bpf_generic_log {
> + char kbuf[BPF_GENERIC_TMP_LOG_SIZE];
> + char __user *ubuf;
> + u32 len_used;
> + u32 len_total;
> +};
> +
> +__printf(2, 3) void bpf_generic_log_write(struct bpf_generic_log *log,
> + const char *fmt, ...);
> +static inline void bpf_generic_log_init(struct bpf_generic_log *log,
> + const struct bpf_generic_user_log *ulog)
> +{
> + log->ubuf = (char __user *) (unsigned long) ulog->log_buf;
> + log->len_total = ulog->log_size;
> + log->len_used = 0;
> +}
> +
> +#define BPF_GENERIC_LOG_WRITE(log, ulog, fmt, ...) do { \
> + const char *____fmt = (fmt); \
> + bpf_generic_log_init(log, ulog); \
> + bpf_generic_log_write(log, ____fmt, ##__VA_ARGS__); \
> +} while (0)
> +
> +#define BPF_GENERIC_ULOG_WRITE(ulog, fmt, ...) do { \
> + struct bpf_generic_log ____log; \
> + BPF_GENERIC_LOG_WRITE(&____log, ulog, fmt, ##__VA_ARGS__); \
> +} while (0)
> +
> #endif /* _LINUX_BPF_H */
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 60a9d59beeabb..34fa334938ba5 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1318,6 +1318,11 @@ struct bpf_stack_build_id {
> };
> };
>
> +struct bpf_generic_user_log {
> + __aligned_u64 log_buf; /* user supplied buffer */
> + __u32 log_size; /* size of user buffer */
> +};
> +
> #define BPF_OBJ_NAME_LEN 16U
>
> union bpf_attr {
> @@ -1544,6 +1549,7 @@ union bpf_attr {
> };
> __u32 attach_type; /* attach type */
> __u32 flags; /* extra flags */
> + struct bpf_generic_user_log log; /* user log */
I think explicit triplet of log_level (should be log_flags, but too
late, probably), log_size, and log_buf is less error prone and more in
sync with other two commands that accept log (BPF_PROG_LOAD and
BPF_BTF_LOAD).
> union {
> __u32 target_btf_id; /* btf_id of target to attach to */
> struct {
> diff --git a/kernel/bpf/log.c b/kernel/bpf/log.c
> index 850494423530e..be56b153bbf0b 100644
> --- a/kernel/bpf/log.c
> +++ b/kernel/bpf/log.c
> @@ -325,3 +325,36 @@ __printf(2, 3) void bpf_log(struct bpf_verifier_log *log,
> va_end(args);
> }
> EXPORT_SYMBOL_GPL(bpf_log);
> +
> +static inline void __bpf_generic_log_write(struct bpf_generic_log *log, const char *fmt,
> + va_list args)
> +{
> + unsigned int n;
> +
> + n = vscnprintf(log->kbuf, BPF_GENERIC_TMP_LOG_SIZE, fmt, args);
> +
> + WARN_ONCE(n >= BPF_GENERIC_TMP_LOG_SIZE - 1,
> + "bpf generic log truncated - local buffer too short\n");
> +
> + n = min(log->len_total - log->len_used - 1, n);
> + log->kbuf[n] = '\0';
> +
> + if (!copy_to_user(log->ubuf + log->len_used, log->kbuf, n + 1))
> + log->len_used += n;
> + else
> + log->ubuf = NULL;
> +}
> +
please see bpf_verifier_vlog() in kernel/bpf/log.c. We don't want to
maintain another (even if light) version of it.
> +__printf(2, 3) void bpf_generic_log_write(struct bpf_generic_log *log,
> + const char *fmt, ...)
> +{
> + va_list args;
> +
> + if (!log->ubuf || !log->len_total)
> + return;
> +
> + va_start(args, fmt);
> + __bpf_generic_log_write(log, fmt, args);
> + va_end(args);
> +}
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