linux-kernel - Re: [PATCH] procfs: don't need a PATH_MAX allocation to hold a string representation of an int

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Date:	Fri, 7 Sep 2012 15:44:16 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Pádraig Brady <P@...igBrady.com>
Cc:	Jeff Layton <jlayton@...chiereds.net>, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] procfs: don't need a PATH_MAX allocation to hold a
 string representation of an int

On Fri, Sep 07, 2012 at 01:58:03PM +0100, Pádraig Brady wrote:
> On 09/07/2012 01:48 PM, Jeff Layton wrote:
> >On Fri,  7 Sep 2012 08:34:53 -0400
> >Jeff Layton<jlayton@...hat.com>  wrote:
> >
> >>Signed-off-by: Jeff Layton<jlayton@...hat.com>
> >>---
> >>  fs/proc/base.c | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/fs/proc/base.c b/fs/proc/base.c
> >>index 1b6c84c..58e801b 100644
> >>--- a/fs/proc/base.c
> >>+++ b/fs/proc/base.c
> >>@@ -2758,7 +2758,8 @@ static void *proc_self_follow_link(struct dentry *dentry, struct nameidata *nd)
> >>  	pid_t tgid = task_tgid_nr_ns(current, ns);
> >>  	char *name = ERR_PTR(-ENOENT);
> >>  	if (tgid) {
> >>-		name = __getname();
> >>+		/* 10 for max length of an int in decimal + NULL terminator */
> >>+		name = kmalloc(11, GFP_KERNEL);
> >
> >		^^^^^
> >Bah...my mistake. This should be "12", since it's possible (though
> >unlikely) that this value could be negative. Is there a better way to
> >express "strlen of max representation of an int in decimal" ?

It'd be nice to have something--I've run across similar mistakes
recently.

> 
> See INT_BUFSIZE_BOUND() in:
> http://git.sv.gnu.org/gitweb/?p=gnulib.git;a=blob;f=lib/intprops.h;hb=HEAD

My lame attempt follows.  It's simpler than Pádraig's but possibly also
stupider.

--b.

commit 15e8c46104e0c1dd6a76d09b55563b6f83b61667
Author: J. Bruce Fields <bfields@...hat.com>
Date:   Wed Aug 15 17:41:47 2012 -0400

    strings: helper for maximum decimal encoding of an unsigned integer
    
    I've seen a couple examples recently where we've gotten this wrong.
    Maybe something like this would help?
    
    Suggested-by: Jim Rees <rees@...ch.edu>
    Signed-off-by: J. Bruce Fields <bfields@...hat.com>

diff --git a/include/linux/string.h b/include/linux/string.h
index ffe0442..3674cf5 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -126,6 +126,12 @@ extern void argv_free(char **argv);
 extern bool sysfs_streq(const char *s1, const char *s2);
 extern int strtobool(const char *s, bool *res);
 
+/*
+ * length of the decimal representation of an unsigned integer.  Just an
+ * approximation, but it's right for types of size 1 to 26 bytes:
+ */
+#define base10len(i) (sizeof(i) * 24 / 10 + 1)
+
 #ifdef CONFIG_BINARY_PRINTF
 int vbin_printf(u32 *bin_buf, size_t size, const char *fmt, va_list args);
 int bstr_printf(char *buf, size_t size, const char *fmt, const u32 *bin_buf);
diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index 2afd2a8..1dcd2b3 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -1409,7 +1409,7 @@ static ssize_t read_flush(struct file *file, char __user *buf,
 			  size_t count, loff_t *ppos,
 			  struct cache_detail *cd)
 {
-	char tbuf[20];
+	char tbuf[base10len(unsigned long) + 2];
 	unsigned long p = *ppos;
 	size_t len;
 
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