linux-kernel - Re: [PATCH] Fix wrong EOF compare

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Date:	Fri, 11 Jan 2013 08:34:56 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Randy Dunlap <rdunlap@...radead.org>
Cc:	Michal Nazarewicz <mina86@...a86.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
	Andy Whitcroft <apw@...dowen.org>,
	Alexander Nyberg <alexn@....su.se>
Subject: Re: [PATCH] Fix wrong EOF compare

Hi Randy,

On Thu, Jan 10, 2013 at 08:12:24AM -0800, Randy Dunlap wrote:
> On 01/10/13 07:26, Michal Nazarewicz wrote:
> > On Thu, Jan 10 2013, Minchan Kim <minchan@...nel.org> wrote:
> >> getc returns "int" so EOF could be -1 but storing getc's return
> >> value to char directly makes the vaule to 255 so below condition
> >> is always false.
> > 
> > Technically, this is implementation defined and I believe on many
> > systems char is signed thus the loop will end on EOF or byte 255.
> > 
> > Either way, my point is the patch is correct, but the comment is not. ;)
> 
> and change spelling of 'vaule' to 'value'

Oops. It was typo when I cooked the patch after testing in my ARM system.

> and test build it please.

Keep in mind. Thanks!

> 
> > 
> > Of course, even better if the function just used fgets(), ie. something
> > like:
> > 
> > int read_block(char *buf, int buf_size, FILE *fin)
> > {
> > 	char *curr = buf, *const buf_end = buf + buf_size;
> > 
> > 	while (buf_end - curr > 1 && fgets(curr, buf_end - curr, fin)) {
> > 		if (*curr == '\n') /* empty line */
> > 			return curr - buf;
> > 		curr += strlen(curr);
> > 	}
> > 
> > 	return -1; /* EOF or no space left in buf. */
> > }
> > 
> > which is much shorter and does not have buffer overflow issues.
> > 
> >> It happens in my ARM system so loop is not ended, then segfaulted.
> >> This patch fixes it.
> >>
> >>                 *curr = getc(fin); // *curr = 255
> >>                 if (*curr == EOF) return -1; // if ( 255 == -1)
> >>
> >> Cc: Mel Gorman <mgorman@...e.de>
> >> Cc: Andy Whitcroft <apw@...dowen.org>
> >> Cc: Alexander Nyberg <alexn@....su.se>
> >> Signed-off-by: Minchan Kim <minchan@...nel.org>
> >> ---
> >>  Documentation/page_owner.c |    6 ++++--
> >>  1 file changed, 4 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/Documentation/page_owner.c b/Documentation/page_owner.c
> >> index f0156e1..b777fb6 100644
> >> --- a/Documentation/page_owner.c
> >> +++ b/Documentation/page_owner.c
> >> @@ -32,12 +32,14 @@ int read_block(char *buf, FILE *fin)
> >>  {
> >>  	int ret = 0;
> >>  	int hit = 0;
> >> +	int vaule;
> >>  	char *curr = buf;
> >>  
> >>  	for (;;) {
> >> -		*curr = getc(fin);
> >> -		if (*curr == EOF) return -1;
> >> +		value = getc(fin);
> >> +		if (value == EOF) return -1;
> >>  
> >> +		*curr = value;
> >>  		ret++;
> >>  		if (*curr == '\n' && hit == 1)
> >>  			return ret - 1;
> > 
> > 
> > 
> 
> 
> -- 
> ~Randy
> 
> --
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-- 
Kind regards,
Minchan Kim
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