linux-kernel - Re: [PATCH 22/28] x86: apm: avoid uninitialized data

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Date:   Tue, 18 Oct 2016 23:35:41 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH 22/28] x86: apm: avoid uninitialized data

On Tue, Oct 18, 2016 at 12:16:10AM +0200, Arnd Bergmann wrote:
> apm_bios_call() can fail, and return a status in its argument
> structure. If that status however is zero during a call from
> apm_get_power_status(), we end up using data that may have
> never been set, as reported by "gcc -Wmaybe-uninitialized":

Userspace *may* already rely on this broken behavior for broken
BIOSes which may leave the return value as 0, ignoring that,
this change makes sense to me given that handling the error
would be better than relying any possible invalid data.

> arch/x86/kernel/apm_32.c: In function ‘apm’:
> arch/x86/kernel/apm_32.c:1729:17: error: ‘bx’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> arch/x86/kernel/apm_32.c:1835:5: error: ‘cx’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> arch/x86/kernel/apm_32.c:1730:17: note: ‘cx’ was declared here
> arch/x86/kernel/apm_32.c:1842:27: error: ‘dx’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> arch/x86/kernel/apm_32.c:1731:17: note: ‘dx’ was declared here
> 
> This changes the function to return "APM_NO_ERROR" here, which
> makes the code more robust to broken BIOS versions, and avoids
> the warning.
> 
> Cc: x86@...nel.org
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Luis R. Rodriguez <mcgrof@...nel.org>

  Luis

> ---
>  arch/x86/kernel/apm_32.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/apm_32.c b/arch/x86/kernel/apm_32.c
> index c7364bd..51287cd 100644
> --- a/arch/x86/kernel/apm_32.c
> +++ b/arch/x86/kernel/apm_32.c
> @@ -1042,8 +1042,11 @@ static int apm_get_power_status(u_short *status, u_short *bat, u_short *life)
>  
>  	if (apm_info.get_power_status_broken)
>  		return APM_32_UNSUPPORTED;
> -	if (apm_bios_call(&call))
> +	if (apm_bios_call(&call)) {
> +		if (!call.err)
> +			return APM_NO_ERROR;
>  		return call.err;
> +	}
>  	*status = call.ebx;
>  	*bat = call.ecx;
>  	if (apm_info.get_power_status_swabinminutes) {
> -- 
> 2.9.0
> 
> 

-- 
Luis Rodriguez, SUSE LINUX GmbH
Maxfeldstrasse 5; D-90409 Nuernberg