linux-kernel - Re: [PATCH] module: Use binary search in lookup

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Message-ID: <4DD14469.5030804@googlemail.com>
Date:	Mon, 16 May 2011 17:36:09 +0200
From:	Dirk Behme <dirk.behme@...glemail.com>
To:	Alessio Igor Bogani <abogani@...nel.org>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	Tim Abbott <tabbott@...lice.com>,
	Anders Kaseorg <andersk@...lice.com>,
	Tim Bird <tim.bird@...sony.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Embedded <linux-embedded@...r.kernel.org>,
	Jason Wessel <jason.wessel@...driver.com>
Subject: Re: [PATCH] module: Use binary search in lookup_symbol()

On 03.05.2011 22:42, Alessio Igor Bogani wrote:
> This work was supported by a hardware donation from the CE Linux Forum.
>
> Signed-off-by: Alessio Igor Bogani<abogani@...nel.org>
> ---
>   kernel/module.c |    6 ++----
>   1 files changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/module.c b/kernel/module.c
> index 6a34337..a1f841e 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2055,10 +2055,8 @@ static const struct kernel_symbol *lookup_symbol(const char *name,
>   	const struct kernel_symbol *stop)
>   {
>   	const struct kernel_symbol *ks = start;
> -	for (; ks<  stop; ks++)
> -		if (strcmp(ks->name, name) == 0)
> -			return ks;
> -	return NULL;
> +	return bsearch(ks->name, start, stop - start,
> +			sizeof(struct kernel_symbol), cmp_name);
>   }

Back porting this patch to a 2.6.34.9 based ARM system fails with an 
Oops at 0x00000004. Debugging shows that both start and stop are 0 in 
this case resulting in ks->name accessing 0x00000004. The original 
code checked for this by 'ks < stop' in the for loop.

So the first idea was that the code should be

if(k < stop)
	return bsearch();
else
	return NULL;

Then, thinking again, results in the question if the first argument of 
bsearch() shouldn't be 'name' rather than 'ks->name'? Then it would be 
the job of cmp_name() to check for start == stop == 0? I.e.

return bsearch(name, ...);

?

Best regards

Dirk
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