linux-kernel - Re: [PATCH] fs/direct-io.c: Fix compilation warning for uninitialized variables

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Date:	Wed, 16 Jul 2014 20:08:41 +0300
From:	Boaz Harrosh <openosd@...il.com>
To:	Christoph Hellwig <hch@...radead.org>, pramod.gurav.etc@...il.com,
	viro@...iv.linux.org.uk, Jason Cooper <jason@...edaemon.net>,
	Markus Mayer <markus.mayer@...aro.org>,
	Paul Bolle <pebolle@...cali.nl>
CC:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/direct-io.c: Fix compilation warning for uninitialized
 variables

On 07/13/2014 02:50 PM, Christoph Hellwig wrote:
> pramod.gurav.etc@...il.com wrote ...
>> diff --git a/fs/direct-io.c b/fs/direct-io.c
>> index 98040ba388ac..c0a9854d2bc7 100644
>> --- a/fs/direct-io.c
>> +++ b/fs/direct-io.c
>> @@ -910,7 +910,8 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
>>  
>>  	while (sdio->block_in_file < sdio->final_block_in_request) {
>>  		struct page *page;
>> -		size_t from, to;
>> +		size_t from = 0;
>> +		size_t to = 0;
>>  		page = dio_get_page(dio, sdio, &from, &to);
>>  		if (IS_ERR(page)) {
>>  			ret = PTR_ERR(page);
> Looks good to me,
> 
> Reviewed-by: Christoph Hellwig <hch@....de>
> 
> Al, can you pick this up?  It's the only warnings in many of my usual
> kernel builds at the moment.
> 
<>

Paul Bolle <pebolle@...cali.nl> wrote ...
<>
> @@ -911,11 +907,15 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
>  	while (sdio->block_in_file < sdio->final_block_in_request) {
>  		struct page *page;
>  		size_t from, to;
> -		page = dio_get_page(dio, sdio, &from, &to);
> +
> +		page = dio_get_page(dio, sdio);
>  		if (IS_ERR(page)) {
>  			ret = PTR_ERR(page);
>  			goto out;
>  		}
> +		from = sdio->head ? 0 : sdio->from;
> +		to = (sdio->head == sdio->tail - 1) ? sdio->to : PAGE_SIZE;
> +		sdio->head++;
>  
>  		while (from < to) {
>  			unsigned this_chunk_bytes;	/* # of bytes mapped */
<>

This is the wrong fix. GCC is wrong here. As shown by Paul Bolle if
you move the from / to set from dio_get_page() to here the warning goes away.

The minimal fix must use uninitialized_var() in this case. See patch below

But I think the proper fix Is the one Paul Bolle <pebolle@...cali.nl> sent (above)


----
From: Boaz Harrosh <boaz@...xistor.com>
Date: Wed, 16 Jul 2014 20:02:29 +0300
Subject: [PATCH] do_direct_IO: Fix compiler warning
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

This fixes
../fs/direct-io.c: In function ‘do_blockdev_direct_IO’:
../fs/direct-io.c:1011:12: warning: ‘from’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    u = (to - from) >> blkbits;
            ^
../fs/direct-io.c:1011:12: warning: ‘to’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    u = (to - from) >> blkbits;

I use: gcc version 4.8.3 20140624 (Red Hat 4.8.3-1)

GCC is wrong here so we should use the uninitialized_var() macro
and not silence it with = 0;

Signed-off-by: Boaz Harrosh <boaz@...xistor.com>
---
 fs/direct-io.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/direct-io.c b/fs/direct-io.c
index 98040ba..156e6f0 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -910,7 +910,8 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
 
 	while (sdio->block_in_file < sdio->final_block_in_request) {
 		struct page *page;
-		size_t from, to;
+		size_t uninitialized_var(from), uninitialized_var(to);
+
 		page = dio_get_page(dio, sdio, &from, &to);
 		if (IS_ERR(page)) {
 			ret = PTR_ERR(page);
-- 
1.9.3

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