linux-kernel - Re: [PATCH 4.19 051/168] dyndbg: fix a BUG_ON in ddebug_describe

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Date:   Tue, 18 Aug 2020 11:51:24 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        jbaron@...mai.com, Jim Cromie <jim.cromie@...il.com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 051/168] dyndbg: fix a BUG_ON in
 ddebug_describe_flags

On Mon 2020-08-17 17:16:22, Greg Kroah-Hartman wrote:
> From: Jim Cromie <jim.cromie@...il.com>
> 
> [ Upstream commit f678ce8cc3cb2ad29df75d8824c74f36398ba871 ]
> 
> ddebug_describe_flags() currently fills a caller provided string buffer,
> after testing its size (also passed) in a BUG_ON.  Fix this by
> replacing them with a known-big-enough string buffer wrapped in a
> struct, and passing that instead.
> 
> Also simplify ddebug_describe_flags() flags parameter from a struct to
> a member in that struct, and hoist the member deref up to the caller.
> This makes the function reusable (soon) where flags are unpacked.

Original code was correct, passing explicit size, this passes strange
structure. BUG_ON can never trigger in the origianl code, so this is
not a bugfix.

Best regards,
								Pavel

> +++ b/lib/dynamic_debug.c
> @@ -85,22 +85,22 @@ static struct { unsigned flag:8; char opt_char; } opt_array[] = {
>  	{ _DPRINTK_FLAGS_NONE, '_' },
>  };
>  
> +struct flagsbuf { char buf[ARRAY_SIZE(opt_array)+1]; };
> +
>  /* format a string into buf[] which describes the _ddebug's flags */
> -static char *ddebug_describe_flags(struct _ddebug *dp, char *buf,
> -				    size_t maxlen)
> +static char *ddebug_describe_flags(unsigned int flags, struct flagsbuf *fb)
>  {
> -	char *p = buf;
> +	char *p = fb->buf;
>  	int i;
>  
> -	BUG_ON(maxlen < 6);
>  	for (i = 0; i < ARRAY_SIZE(opt_array); ++i)
> -		if (dp->flags & opt_array[i].flag)
> +		if (flags & opt_array[i].flag)
>  			*p++ = opt_array[i].opt_char;
> -	if (p == buf)
> +	if (p == fb->buf)
>  		*p++ = '_';
>  	*p = '\0';
>  
> -	return buf;
> +	return fb->buf;
>  }
>  
>  #define vpr_info(fmt, ...)					\
> @@ -142,7 +142,7 @@ static int ddebug_change(const struct ddebug_query *query,
>  	struct ddebug_table *dt;
>  	unsigned int newflags;
>  	unsigned int nfound = 0;
> -	char flagbuf[10];
> +	struct flagsbuf fbuf;
>  
>  	/* search for matching ddebugs */
>  	mutex_lock(&ddebug_lock);
> @@ -199,8 +199,7 @@ static int ddebug_change(const struct ddebug_query *query,
>  			vpr_info("changed %s:%d [%s]%s =%s\n",
>  				 trim_prefix(dp->filename), dp->lineno,
>  				 dt->mod_name, dp->function,
> -				 ddebug_describe_flags(dp, flagbuf,
> -						       sizeof(flagbuf)));
> +				 ddebug_describe_flags(dp->flags, &fbuf));
>  		}
>  	}
>  	mutex_unlock(&ddebug_lock);
> @@ -779,7 +778,7 @@ static int ddebug_proc_show(struct seq_file *m, void *p)
>  {
>  	struct ddebug_iter *iter = m->private;
>  	struct _ddebug *dp = p;
> -	char flagsbuf[10];
> +	struct flagsbuf flags;
>  
>  	vpr_info("called m=%p p=%p\n", m, p);
>  
> @@ -792,7 +791,7 @@ static int ddebug_proc_show(struct seq_file *m, void *p)
>  	seq_printf(m, "%s:%u [%s]%s =%s \"",
>  		   trim_prefix(dp->filename), dp->lineno,
>  		   iter->table->mod_name, dp->function,
> -		   ddebug_describe_flags(dp, flagsbuf, sizeof(flagsbuf)));
> +		   ddebug_describe_flags(dp->flags, &flags));
>  	seq_escape(m, dp->format, "\t\r\n\"");
>  	seq_puts(m, "\"\n");
>  

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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