linux-kernel - Re: [PATCH 2/2] drm/i915: Fix incorrect error handling in shmem

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Message-ID: <aJtKbg28QMPgvunP@intel.com>
Date: Tue, 12 Aug 2025 10:06:38 -0400
From: Rodrigo Vivi <rodrigo.vivi@...el.com>
To: 陈涛涛 Taotao Chen <chentaotao@...iglobal.com>
CC: "jani.nikula@...ux.intel.com" <jani.nikula@...ux.intel.com>,
	"joonas.lahtinen@...ux.intel.com" <joonas.lahtinen@...ux.intel.com>,
	"tursulin@...ulin.net" <tursulin@...ulin.net>, "airlied@...il.com"
	<airlied@...il.com>, "simona@...ll.ch" <simona@...ll.ch>,
	"oe-lkp@...ts.linux.dev" <oe-lkp@...ts.linux.dev>, "lkp@...el.com"
	<lkp@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "intel-gfx@...ts.freedesktop.org"
	<intel-gfx@...ts.freedesktop.org>, "brauner@...nel.org" <brauner@...nel.org>,
	"oliver.sang@...el.com" <oliver.sang@...el.com>
Subject: Re: [PATCH 2/2] drm/i915: Fix incorrect error handling in
 shmem_pwrite()

On Mon, Aug 11, 2025 at 09:31:00AM +0000, 陈涛涛 Taotao Chen wrote:
> From: Taotao Chen <chentaotao@...iglobal.com>
> 
> shmem_pwrite() currently checks for short writes before negative error
> codes, which can overwrite real errors (e.g., -EFBIG) with -EIO.
> Reorder the checks to return negative errors first, then handle short
> writes.
> 
> Fixes: 048832a3f400 ("drm/i915: Refactor shmem_pwrite() to use kiocb and write_iter")
> Signed-off-by: Taotao Chen <chentaotao@...iglobal.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> index 2b53aad915f5..702532eef207 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> @@ -441,12 +441,12 @@ shmem_pwrite(struct drm_i915_gem_object *obj,
>  	written = file->f_op->write_iter(&kiocb, &iter);
>  	BUG_ON(written == -EIOCBQUEUED);
>  
> -	if (written != size)
> -		return -EIO;
> -
>  	if (written < 0)
>  		return written;
>  
> +	if (written != size)
> +		return -EIO;

That's awkward...

I mean, you are right that we cannot overwrite what is returned from the
write_iter function. But perhaps this != check here should be before?

Or it at least deserves a comment in the code telling what's the intent
here. why != size is -EIO... but it was already written :/

> +
>  	return 0;
>  }
>  
> -- 
> 2.34.1