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Date: Wed, 5 Jul 2023 16:39:25 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Leon Hwang <hffilwlqm@...il.com>, ast@...nel.org
Cc: john.fastabend@...il.com, andrii@...nel.org, martin.lau@...ux.dev,
song@...nel.org, yhs@...com, kpsingh@...nel.org, sdf@...gle.com,
haoluo@...gle.com, jolsa@...nel.org, davem@...emloft.net,
edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
hawk@...nel.org, tangyeechou@...il.com, kernel-patches-bot@...com,
bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: Introduce bpf generic log
On 7/5/23 3:20 PM, Leon Hwang 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@...il.com>
> ---
> 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(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index f58895830..fd63f4a76 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -3077,4 +3077,34 @@ 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)
> +
Could we generalize the bpf_verifier_log infra and reuse bpf_log() helper
instead of adding something new?
> #endif /* _LINUX_BPF_H */
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 60a9d59be..34fa33493 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 */
You cannot add this here, this breaks user space, you would have to
ad this under a xdp specific section inside the union.
> 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 850494423..be56b153b 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;
> +}
> +
> +__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);
> +}
> +EXPORT_SYMBOL_GPL(bpf_generic_log_write);
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 69a3e5446..e933809c0 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -9409,12 +9409,20 @@ static const struct bpf_link_ops bpf_xdp_link_lops = {
> .update_prog = bpf_xdp_link_update,
> };
>
> +static inline void bpf_xdp_link_log(const union bpf_attr *attr, struct netlink_ext_ack *extack)
> +{
> + const struct bpf_generic_user_log *ulog = &attr->link_create.log;
> +
> + BPF_GENERIC_ULOG_WRITE(ulog, extack->_msg);
> +}
> +
> int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> {
> struct net *net = current->nsproxy->net_ns;
> struct bpf_link_primer link_primer;
> struct bpf_xdp_link *link;
> struct net_device *dev;
> + struct netlink_ext_ack extack;
> int err, fd;
>
> rtnl_lock();
> @@ -9440,12 +9448,13 @@ int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> goto unlock;
> }
>
> - err = dev_xdp_attach_link(dev, NULL, link);
> + err = dev_xdp_attach_link(dev, &extack, link);
> rtnl_unlock();
>
> if (err) {
> link->dev = NULL;
> bpf_link_cleanup(&link_primer);
> + bpf_xdp_link_log(attr, &extack);
> goto out_put_dev;
> }
Agree that this is a useful facility to have and propagate back here.
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 60a9d59be..34fa33493 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/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 */
> union {
> __u32 target_btf_id; /* btf_id of target to attach to */
> struct {
>
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