Re: [PATCH RFC] vfs: always log mount API fs context messages to dmesg

[Bill O'Donnell <bodonnel@redhat.com>]

On Thu, Feb 22, 2024 at 09:22:52AM -0600, Eric Sandeen wrote:
> As filesystems are converted to the new mount API, informational messages,
> errors, and warnings are being routed through infof, errorf, and warnf
> type functions provided by the mount API, which places these messages in
> the log buffer associated with the filesystem context rather than
> in the kernel log / dmesg.
> 
> However, userspace is not yet extracting these messages, so they are
> essentially getting lost. mount(8) still refers the user to dmesg(1)
> on failure.
> 
> At least for now, modify logfc() to always log these messages to dmesg
> as well as to the fileystem context. This way we can continue converting
> filesystems to the new mount API interfaces without worrying about losing
> this information until userspace can retrieve it.
> 
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>

Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>

> ---
> 
> A few considerations/concerns:
> 
> * viro suggested that maybe this should be conditional - possibly config
> 
> * systemd is currently probing with a dummy mount option which will
>   generate noise, see
>   https://github.com/systemd/systemd/blob/main/src/basic/mountpoint-util.c#L759
>   i.e. - 
>   [   10.689256] proc: Unknown parameter 'adefinitelynotexistingmountoption'
>   [   10.801045] tmpfs: Unknown parameter 'adefinitelynotexistingmountoption'
>   [   11.119431] proc: Unknown parameter 'adefinitelynotexistingmountoption'
>   [   11.692032] proc: Unknown parameter 'adefinitelynotexistingmountoption'
> 
> * will this generate other dmesg noise in general if the mount api messages
>   are more noisy in general? (spot-checking old conversions, I don't think so.)
> 
>  fs/fs_context.c | 38 ++++++++++++++++++++------------------
>  1 file changed, 20 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/fs_context.c b/fs/fs_context.c
> index 98589aae5208..3c78b99d5cae 100644
> --- a/fs/fs_context.c
> +++ b/fs/fs_context.c
> @@ -427,8 +427,8 @@ struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc)
>  EXPORT_SYMBOL(vfs_dup_fs_context);
>  
>  /**
> - * logfc - Log a message to a filesystem context
> - * @log: The filesystem context to log to, or NULL to use printk.
> + * logfc - Log a message to dmesg and optionally a filesystem context
> + * @log: The filesystem context to log to, or NULL to use printk alone
>   * @prefix: A string to prefix the output with, or NULL.
>   * @level: 'w' for a warning, 'e' for an error.  Anything else is a notice.
>   * @fmt: The format of the buffer.
> @@ -439,22 +439,24 @@ void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt,
>  	struct va_format vaf = {.fmt = fmt, .va = &va};
>  
>  	va_start(va, fmt);
> -	if (!log) {
> -		switch (level) {
> -		case 'w':
> -			printk(KERN_WARNING "%s%s%pV\n", prefix ? prefix : "",
> -						prefix ? ": " : "", &vaf);
> -			break;
> -		case 'e':
> -			printk(KERN_ERR "%s%s%pV\n", prefix ? prefix : "",
> -						prefix ? ": " : "", &vaf);
> -			break;
> -		default:
> -			printk(KERN_NOTICE "%s%s%pV\n", prefix ? prefix : "",
> -						prefix ? ": " : "", &vaf);
> -			break;
> -		}
> -	} else {
> +	switch (level) {
> +	case 'w':
> +		printk(KERN_WARNING "%s%s%pV\n", prefix ? prefix : "",
> +					prefix ? ": " : "", &vaf);
> +		break;
> +	case 'e':
> +		printk(KERN_ERR "%s%s%pV\n", prefix ? prefix : "",
> +					prefix ? ": " : "", &vaf);
> +		break;
> +	default:
> +		printk(KERN_NOTICE "%s%s%pV\n", prefix ? prefix : "",
> +					prefix ? ": " : "", &vaf);
> +		break;
> +	}
> +	va_end(va);
> +
> +	va_start(va, fmt);
> +	if (log) {
>  		unsigned int logsize = ARRAY_SIZE(log->buffer);
>  		u8 index;
>  		char *q = kasprintf(GFP_KERNEL, "%c %s%s%pV\n", level,
> -- 
> 2.43.0
>