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SubjectRe: [PATCH] printk: inject caller information into the body of message
On Wed, 12 Sep 2018 15:53:07 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:

> I scanned some of Linus' emails, and skimmed through previous discussions
> on this topic. Let me quote Linus:
>
> :
> : My preference as a user is actually to just have a dynamically
> : re-sizable buffer (that's pretty much what I've done in *every* single
> : user space project I've had in the last decade), but because some
> : users might have atomicity issues I do suspect that we should just use
> : a stack buffer.
> :
> : And then perhaps say that the buffer size has to be capped at 80 characters.
> :
> : Because if you're printing more than 80 characters and expecting it
> : all to fit on a line, you're doing something else wrong anyway.
> :
> : And hide it not as a explicit "char buffer[80]]" allocation, but as a
> : "struct line_buffer" or similar, so that
> :
> : (a) people don't get the line size wrong
> :
> : (b) the buffering code can add a few fields for length etc in there too
> :
> : Introduce a few helper functions for it:
> :
> : init_line_buffer(&buf);
> : print_line(&buf, fmt, args);
> : vprint_line(&buf, fmt, vararg);
> : finish_line(&buf);
> :

This sounds like seq_buf to me.

>
>
>
> And this is, basically, what I have attached to this email. It's very
> simple and very short. And I think this is what Linus wanted us to do.
>
> - usage example
>
> DEFINE_PR_LINE(KERN_ERR, pl);
>
> pr_line(&pl, "Hello, %s!\n", "buffer");
> pr_line(&pl, "%s", "OK.\n");
> pr_line(&pl, "Goodbye, %s", "buffer");
> pr_line(&pl, "\n");
>
> dmesg | tail
>
> [ 69.908542] Hello, buffer!
> [ 69.908544] OK.
> [ 69.908545] Goodbye, buffer
>
>
> - pr_cont-like usage
>
> DEFINE_PR_LINE(KERN_ERR, pl);
>
> pr_line(&pl,"%d ", 1);
> pr_line(&pl,"%d ", 3);
> pr_line(&pl,"%d ", 5);
> pr_line(&pl,"%d ", 7);
> pr_line(&pl,"%d\n", 9);
>
> dmesg | tail
>
> [ 69.908546] 1 3 5 7 9
>
>
> - An explicit, aux buffer // output should be truncated
>
> char buf[16];
> DEFINE_PR_LINE_BUF(KERN_ERR, ps, buf, sizeof(buf));
>
> pr_line(&ps, "Test test test test test test test test test\n");
> pr_line(&ps, "\n");
>
>
> dmesg | tail
>
> [ 69.908547] Test test test ** truncated **
>
>
> Opinions? Will this work for us?
>
> ====
>
> >From 7fd8407e0081d8979f08dec48e88364d6210b4ab Mon Sep 17 00:00:00 2001
> From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Subject: [PATCH] printk: add pr_line buffering API
>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
> include/linux/printk.h | 63 ++++++++++++++++++++++++++++++++++++++++++
> kernel/printk/printk.c | 55 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 118 insertions(+)
>
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index cf3eccfe1543..fc5f11c7579c 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -157,6 +157,15 @@ static inline void printk_nmi_direct_enter(void) { }
> static inline void printk_nmi_direct_exit(void) { }
> #endif /* PRINTK_NMI */
>
> +#define PRINTK_PR_LINE_BUF_SZ 80
> +
> +struct pr_line {
> + char *buffer;
> + int size;
> + int len;
> + char *level;
> +};

Can you look at implementing this with using a seq_buf?

-- Steve

> +
> #ifdef CONFIG_PRINTK
> asmlinkage __printf(5, 0)
> int vprintk_emit(int facility, int level,
> @@ -209,6 +218,30 @@ extern asmlinkage void dump_stack(void) __cold;
> extern void printk_safe_init(void);
> extern void printk_safe_flush(void);
> extern void printk_safe_flush_on_panic(void);
> +
> +#define DEFINE_PR_LINE(lev, name) \
> + char __pr_line_buf[PRINTK_PR_LINE_BUF_SZ]; \
> + struct pr_line name = { \
> + .buffer = __pr_line_buf, \
> + .size = PRINTK_PR_LINE_BUF_SZ, \
> + .len = 0, \
> + .level = lev, \
> + }
> +
> +#define DEFINE_PR_LINE_BUF(lev, name, buf, sz) \
> + struct pr_line name = { \
> + .buffer = buf, \
> + .size = (sz), \
> + .len = 0, \
> + .level = lev, \
> + }
> +
> +extern __printf(2, 3)
> +int pr_line(struct pr_line *pl, const char *fmt, ...);
> +extern __printf(2, 0)
> +int vpr_line(struct pr_line *pl, const char *fmt, va_list args);
> +extern void pr_line_flush(struct pr_line *pl);
> +
> #else
> static inline __printf(1, 0)
> int vprintk(const char *s, va_list args)
> @@ -284,6 +317,36 @@ static inline void printk_safe_flush(void)
> static inline void printk_safe_flush_on_panic(void)
> {
> }
> +
> +#define DEFINE_PR_LINE(lev, name) \
> + struct pr_line name = { \
> + .buffer = NULL, \
> + .size = 0, \
> + .len = 0, \
> + .level = lev, \
> + }
> +
> +#define DEFINE_PR_LINE_BUF(lev, name, buf, sz) \
> + struct pr_line name = { \
> + .buffer = buf, \
> + .size = 0, \
> + .len = 0, \
> + .level = lev, \
> + }
> +
> +static inline __printf(2, 3)
> +int pr_line(struct pr_line *pl, const char *fmt, ...)
> +{
> + return 0;
> +}
> +static inline __printf(2, 0)
> +int vpr_line(struct pr_line *pl, const char *fmt, va_list args)
> +{
> + return 0;
> +}
> +static inline void pr_line_flush(struct pr_line *pl)
> +{
> +}
> #endif
>
> extern int kptr_restrict;
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index fd6f8ed28e01..daeb41a57929 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2004,6 +2004,61 @@ asmlinkage __visible int printk(const char *fmt, ...)
> }
> EXPORT_SYMBOL(printk);
>
> +#define PR_LINE_TRUNCATED_MSG "** truncated **\n"
> +
> +int vpr_line(struct pr_line *pl, const char *fmt, va_list args)
> +{
> + int len;
> +
> + if (unlikely(pl->size >= LOG_LINE_MAX))
> + pl->size = LOG_LINE_MAX - sizeof(PR_LINE_TRUNCATED_MSG);
> +
> + if (fmt[0] == '\n') {
> + pr_line_flush(pl);
> + return 0;
> + }
> +
> + if (pl->len >= pl->size)
> + return -1;
> +
> + len = vsnprintf(pl->buffer + pl->len, pl->size - pl->len, fmt, args);
> + if (pl->len + len >= pl->size) {
> + pl->len = pl->size + 1;
> + return -1;
> + }
> +
> + pl->len += len;
> + if (pl->len && pl->buffer[pl->len - 1] == '\n')
> + pr_line_flush(pl);
> + return 0;
> +}
> +EXPORT_SYMBOL(vpr_line);
> +
> +int pr_line(struct pr_line *pl, const char *fmt, ...)
> +{
> + va_list ap;
> + int ret;
> +
> + va_start(ap, fmt);
> + ret = vpr_line(pl, fmt, ap);
> + va_end(ap);
> + return ret;
> +}
> +EXPORT_SYMBOL(pr_line);
> +
> +void pr_line_flush(struct pr_line *pl)
> +{
> + if (!pl->len)
> + return;
> +
> + if (pl->len < pl->size)
> + printk("%s%.*s", pl->level, pl->len, pl->buffer);
> + else
> + printk("%s%.*s%s", pl->level, pl->len, pl->buffer,
> + PR_LINE_TRUNCATED_MSG);
> + pl->len = 0;
> +}
> +EXPORT_SYMBOL(pr_line_flush);
> #else /* CONFIG_PRINTK */
>
> #define LOG_LINE_MAX 0

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