[PATCH] scatterlist: don't BUG when we can trivially return a proper error.

[PATCH] scatterlist: don't BUG when we can trivially return a proper error.

[Nick Bowler]

There is absolutely no reason to crash the kernel when we have a
perfectly good return value already available to use for conveying
failure status.

Let's return an error code instead of crashing the kernel: that sounds
like a much better plan.

Signed-off-by: Nick Bowler <nbowler@elliptictech.com>
---
 lib/scatterlist.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 3675452b23ca..11ecaf000696 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -248,7 +248,8 @@ int __sg_alloc_table(struct sg_table *table, unsigned int nents,
 	unsigned int left;
 
 #ifndef ARCH_HAS_SG_CHAIN
-	BUG_ON(nents > max_ents);
+	if (WARN_ON_ONCE(nents > max_ents))
+		return -E2BIG;
 #endif
 
 	memset(table, 0, sizeof(*table));
-- 
1.7.8.6

Re: [PATCH] scatterlist: don't BUG when we can trivially return a proper error.

[Andrew Morton]

On Thu,  1 Nov 2012 15:03:00 -0400
Nick Bowler <nbowler@elliptictech.com> wrote:

> There is absolutely no reason to crash the kernel when we have a
> perfectly good return value already available to use for conveying
> failure status.

Yes, I suppose that's true.  I don't see a case for BUGging the kernel
here.

> Let's return an error code instead of crashing the kernel: that sounds
> like a much better plan.
> 
> Signed-off-by: Nick Bowler <nbowler@elliptictech.com>
> ---
>  lib/scatterlist.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index 3675452b23ca..11ecaf000696 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -248,7 +248,8 @@ int __sg_alloc_table(struct sg_table *table, unsigned int nents,
>  	unsigned int left;
>  
>  #ifndef ARCH_HAS_SG_CHAIN
> -	BUG_ON(nents > max_ents);
> +	if (WARN_ON_ONCE(nents > max_ents))
> +		return -E2BIG;
>  #endif

OK, pet peeve: if this E2BIG gets returned to userspace, our poor user
will look it up and see "Argument list too long; used when the
arguments passed to a new program being executed with one of the exec
functions occupy too much memory space".  He then gets to spend half a
day reviewing his code's exec() callsites!

See?  Although the error's name sounds like a nice match to the
internal state, it isn't really a match at all and our use of it is
misleading.

Unfortunately there is no EKERNELSCREWEDUP, so we usually use EINVAL.

Re: [PATCH] scatterlist: don't BUG when we can trivially return a proper error.

[Nick Bowler]

On 2012-11-14 13:05 -0800, Andrew Morton wrote:
> On Thu,  1 Nov 2012 15:03:00 -0400
> Nick Bowler <nbowler@elliptictech.com> wrote:
> 
> > There is absolutely no reason to crash the kernel when we have a
> > perfectly good return value already available to use for conveying
> > failure status.
> 
> Yes, I suppose that's true.  I don't see a case for BUGging the kernel
> here.
[...]
> > -	BUG_ON(nents > max_ents);
> > +	if (WARN_ON_ONCE(nents > max_ents))
> > +		return -E2BIG;
> >  #endif
> 
> OK, pet peeve: if this E2BIG gets returned to userspace, our poor user
> will look it up and see "Argument list too long; used when the
> arguments passed to a new program being executed with one of the exec
> functions occupy too much memory space".  He then gets to spend half a
> day reviewing his code's exec() callsites!
> 
> See?  Although the error's name sounds like a nice match to the
> internal state, it isn't really a match at all and our use of it is
> misleading.
> 
> Unfortunately there is no EKERNELSCREWEDUP,

Well, maybe we should add it! :P

> so we usually use EINVAL.

Fair enough.  I will prepare v2.  But perhaps EOPNOTSUPP would be a
better fit?

Thanks,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)

Re: [PATCH] scatterlist: don't BUG when we can trivially return a proper error.

[richard -rw- weinberger]

On Wed, Nov 14, 2012 at 11:15 PM, Nick Bowler <nbowler@elliptictech.com> wrote:
> On 2012-11-14 13:05 -0800, Andrew Morton wrote:
>> On Thu,  1 Nov 2012 15:03:00 -0400
>> Nick Bowler <nbowler@elliptictech.com> wrote:
>>
>> > There is absolutely no reason to crash the kernel when we have a
>> > perfectly good return value already available to use for conveying
>> > failure status.
>>
>> Yes, I suppose that's true.  I don't see a case for BUGging the kernel
>> here.
> [...]
>> > -   BUG_ON(nents > max_ents);
>> > +   if (WARN_ON_ONCE(nents > max_ents))
>> > +           return -E2BIG;
>> >  #endif
>>
>> OK, pet peeve: if this E2BIG gets returned to userspace, our poor user
>> will look it up and see "Argument list too long; used when the
>> arguments passed to a new program being executed with one of the exec
>> functions occupy too much memory space".  He then gets to spend half a
>> day reviewing his code's exec() callsites!
>>
>> See?  Although the error's name sounds like a nice match to the
>> internal state, it isn't really a match at all and our use of it is
>> misleading.
>>
>> Unfortunately there is no EKERNELSCREWEDUP,
>
> Well, maybe we should add it! :P
>
>> so we usually use EINVAL.
>
> Fair enough.  I will prepare v2.  But perhaps EOPNOTSUPP would be a
> better fit?

IMHO this would be even more confusing...

-- 
Thanks,
//richard

Re: [PATCH] scatterlist: don't BUG when we can trivially return a proper error.

[Andrew Morton]

On Wed, 14 Nov 2012 17:15:44 -0500
Nick Bowler <nbowler@elliptictech.com> wrote:

> > Unfortunately there is no EKERNELSCREWEDUP,
> 
> Well, maybe we should add it! :P

Well.  Given how frequently it will be used, it would need to be a much
shorter identifier.  Perhaps "EEK".  Or just "E".

> > so we usually use EINVAL.
> 
> Fair enough.  I will prepare v2.  But perhaps EOPNOTSUPP would be a
> better fit?

I have queued a delta:

From: Andrew Morton <akpm@linux-foundation.org>
Subject: scatterlist-dont-bug-when-we-can-trivially-return-a-proper-error-fix

s/E2BIG/EINVAL/

Cc: Nick Bowler <nbowler@elliptictech.com>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/scatterlist.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN lib/scatterlist.c~scatterlist-dont-bug-when-we-can-trivially-return-a-proper-error-fix lib/scatterlist.c
--- a/lib/scatterlist.c~scatterlist-dont-bug-when-we-can-trivially-return-a-proper-error-fix
+++ a/lib/scatterlist.c
@@ -249,7 +249,7 @@ int __sg_alloc_table(struct sg_table *ta
 
 #ifndef ARCH_HAS_SG_CHAIN
 	if (WARN_ON_ONCE(nents > max_ents))
-		return -E2BIG;
+		return -EINVAL;
 #endif
 
 	memset(table, 0, sizeof(*table));
_