[PATCH v2] fail dentry revalidation after namespace change

[PATCH v2] fail dentry revalidation after namespace change

[Glauber Costa]

When we change the namespace tag of a sysfs entry, the associated dentry
is still kept around. readdir() will work correctly and not display the
old entries, but open() will still succeed, so will reads and writes.

This will no longer happen if sysfs is remounted, hinting that this is a
cache-related problem.

I am using the following sequence to demonstrate that:

shell1:
ip link add type veth
unshare -nm

shell2:
ip link set veth1 <pid_of_shell_1>
cat /sys/devices/virtual/net/veth1/ifindex

Before that patch, this will succeed (fail to fail). After it, it will
correctly return an error. Differently from a normal rename, which we
handle fine, changing the object namespace will keep it's path intact.
So this check seems necessary as well.

[ v2: get type from parent, as suggested by Eric Biederman ]

Signed-off-by: Glauber Costa <glommer@parallels.com>
CC: Tejun Heo <tj@kernel.org>
CC: Eric W. Biederman <ebiederm@xmission.com>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/sysfs/dir.c |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index e6bb9b2..c0bf38a 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -307,6 +307,7 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
 {
 	struct sysfs_dirent *sd;
 	int is_dir;
+	int type;
 
 	if (nd->flags & LOOKUP_RCU)
 		return -ECHILD;
@@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
 	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
 		goto out_bad;
 
+	/* The sysfs dirent has been moved to a different namespace */
+	type = KOBJ_NS_TYPE_NONE;
+	if (sd->s_parent)
+		type = sysfs_ns_type(sd->s_parent);
+	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
+		goto out_bad;
+
 	mutex_unlock(&sysfs_mutex);
 out_valid:
 	return 1;
-- 
1.7.10.4

Re: [PATCH v2] fail dentry revalidation after namespace change

[Eric W. Biederman]

Glauber Costa <glommer@parallels.com> writes:

> When we change the namespace tag of a sysfs entry, the associated dentry
> is still kept around. readdir() will work correctly and not display the
> old entries, but open() will still succeed, so will reads and writes.

Note reads and writes of file handles open before the move should
continue to work.

> This will no longer happen if sysfs is remounted, hinting that this is a
> cache-related problem.
>
> I am using the following sequence to demonstrate that:
>
> shell1:
> ip link add type veth
> unshare -nm
>
> shell2:
> ip link set veth1 <pid_of_shell_1>
> cat /sys/devices/virtual/net/veth1/ifindex
>
> Before that patch, this will succeed (fail to fail). After it, it will
> correctly return an error. Differently from a normal rename, which we
> handle fine, changing the object namespace will keep it's path intact.
> So this check seems necessary as well.
>
> [ v2: get type from parent, as suggested by Eric Biederman ]

Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>

> Signed-off-by: Glauber Costa <glommer@parallels.com>
> CC: Tejun Heo <tj@kernel.org>
> CC: Eric W. Biederman <ebiederm@xmission.com>
> CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  fs/sysfs/dir.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
> index e6bb9b2..c0bf38a 100644
> --- a/fs/sysfs/dir.c
> +++ b/fs/sysfs/dir.c
> @@ -307,6 +307,7 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>  {
>  	struct sysfs_dirent *sd;
>  	int is_dir;
> +	int type;
>  
>  	if (nd->flags & LOOKUP_RCU)
>  		return -ECHILD;
> @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
>  		goto out_bad;
>  
> +	/* The sysfs dirent has been moved to a different namespace */
> +	type = KOBJ_NS_TYPE_NONE;
> +	if (sd->s_parent)
> +		type = sysfs_ns_type(sd->s_parent);
> +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
> +		goto out_bad;
> +
>  	mutex_unlock(&sysfs_mutex);
>  out_valid:
>  	return 1;

Re: [PATCH v2] fail dentry revalidation after namespace change

[Glauber Costa]

On 07/06/2012 01:37 PM, Eric W. Biederman wrote:
> Glauber Costa <glommer@parallels.com> writes:
> 
>> When we change the namespace tag of a sysfs entry, the associated dentry
>> is still kept around. readdir() will work correctly and not display the
>> old entries, but open() will still succeed, so will reads and writes.
> 
> Note reads and writes of file handles open before the move should
> continue to work.

Well, yes. But do you see it as a big problem?

This can probably be fixed as well, but I foresee a big hackishness in
the way =p

Re: [PATCH v2] fail dentry revalidation after namespace change

[Eric W. Biederman]

Glauber Costa <glommer@parallels.com> writes:

> On 07/06/2012 01:37 PM, Eric W. Biederman wrote:
>> Glauber Costa <glommer@parallels.com> writes:
>> 
>>> When we change the namespace tag of a sysfs entry, the associated dentry
>>> is still kept around. readdir() will work correctly and not display the
>>> old entries, but open() will still succeed, so will reads and writes.
>> 
>> Note reads and writes of file handles open before the move should
>> continue to work.
>
> Well, yes. But do you see it as a big problem?
>
> This can probably be fixed as well, but I foresee a big hackishness in
> the way =p

At the moment it looks like a feature.

The only reason we bounce between different instances of sysfs is
because of the unfortunate sysfs directory layout that we need
to remain compatible with.

But I don't see it making much of a difference either way.

Eric

Re: [PATCH v2] fail dentry revalidation after namespace change

[Andrew Morton]

On Fri,  6 Jul 2012 13:09:07 +0400
Glauber Costa <glommer@parallels.com> wrote:

> When we change the namespace tag of a sysfs entry, the associated dentry
> is still kept around. readdir() will work correctly and not display the
> old entries, but open() will still succeed, so will reads and writes.
> 
> This will no longer happen if sysfs is remounted, hinting that this is a
> cache-related problem.
> 
> I am using the following sequence to demonstrate that:
> 
> shell1:
> ip link add type veth
> unshare -nm
> 
> shell2:
> ip link set veth1 <pid_of_shell_1>
> cat /sys/devices/virtual/net/veth1/ifindex
> 
> Before that patch, this will succeed (fail to fail). After it, it will
> correctly return an error. Differently from a normal rename, which we
> handle fine, changing the object namespace will keep it's path intact.
> So this check seems necessary as well.
> 
> ...
>
> --- a/fs/sysfs/dir.c
> +++ b/fs/sysfs/dir.c
> @@ -307,6 +307,7 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>  {
>  	struct sysfs_dirent *sd;
>  	int is_dir;
> +	int type;
>  
>  	if (nd->flags & LOOKUP_RCU)
>  		return -ECHILD;
> @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
>  		goto out_bad;
>  
> +	/* The sysfs dirent has been moved to a different namespace */
> +	type = KOBJ_NS_TYPE_NONE;
> +	if (sd->s_parent)
> +		type = sysfs_ns_type(sd->s_parent);
> +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))

eww, the code is assuming that KOBJ_NS_TYPE_NONE has a value of zero. 
Don't do that; it smells bad.

I renamed my version of this patch to "sysfs: fail dentry revalidation
after namespace change", as carefully explained in section 15 of the
excellent Documentation/SubmittingPatches, then queued this:


From: Andrew Morton <akpm@linux-foundation.org>
Subject: sysfs-fail-dentry-revalidation-after-namespace-change-fix

don't assume that KOBJ_NS_TYPE_NONE==0.  Also save a test-n-branch.

Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/sysfs/dir.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff -puN fs/sysfs/dir.c~sysfs-fail-dentry-revalidation-after-namespace-change-fix fs/sysfs/dir.c
--- a/fs/sysfs/dir.c~sysfs-fail-dentry-revalidation-after-namespace-change-fix
+++ a/fs/sysfs/dir.c
@@ -329,10 +329,12 @@ static int sysfs_dentry_revalidate(struc
 
 	/* The sysfs dirent has been moved to a different namespace */
 	type = KOBJ_NS_TYPE_NONE;
-	if (sd->s_parent)
+	if (sd->s_parent) {
 		type = sysfs_ns_type(sd->s_parent);
-	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
-		goto out_bad;
+		if (type != KOBJ_NS_TYPE_NONE &&
+				sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns)
+			goto out_bad;
+	}
 
 	mutex_unlock(&sysfs_mutex);
 out_valid:
_

Re: [PATCH v2] fail dentry revalidation after namespace change

[Serge Hallyn]

Quoting Andrew Morton (akpm@linux-foundation.org):
> On Fri,  6 Jul 2012 13:09:07 +0400
> Glauber Costa <glommer@parallels.com> wrote:
> 
> > When we change the namespace tag of a sysfs entry, the associated dentry
> > is still kept around. readdir() will work correctly and not display the
> > old entries, but open() will still succeed, so will reads and writes.
> > 
> > This will no longer happen if sysfs is remounted, hinting that this is a
> > cache-related problem.
> > 
> > I am using the following sequence to demonstrate that:
> > 
> > shell1:
> > ip link add type veth
> > unshare -nm
> > 
> > shell2:
> > ip link set veth1 <pid_of_shell_1>
> > cat /sys/devices/virtual/net/veth1/ifindex
> > 
> > Before that patch, this will succeed (fail to fail). After it, it will
> > correctly return an error. Differently from a normal rename, which we
> > handle fine, changing the object namespace will keep it's path intact.
> > So this check seems necessary as well.
> > 
> > ...
> >
> > --- a/fs/sysfs/dir.c
> > +++ b/fs/sysfs/dir.c
> > @@ -307,6 +307,7 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
> >  {
> >  	struct sysfs_dirent *sd;
> >  	int is_dir;
> > +	int type;
> >  
> >  	if (nd->flags & LOOKUP_RCU)
> >  		return -ECHILD;
> > @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
> >  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
> >  		goto out_bad;
> >  
> > +	/* The sysfs dirent has been moved to a different namespace */
> > +	type = KOBJ_NS_TYPE_NONE;
> > +	if (sd->s_parent)
> > +		type = sysfs_ns_type(sd->s_parent);
> > +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
> 
> eww, the code is assuming that KOBJ_NS_TYPE_NONE has a value of zero. 
> Don't do that; it smells bad.
> 
> I renamed my version of this patch to "sysfs: fail dentry revalidation
> after namespace change", as carefully explained in section 15 of the
> excellent Documentation/SubmittingPatches, then queued this:
> 
> 
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: sysfs-fail-dentry-revalidation-after-namespace-change-fix
> 
> don't assume that KOBJ_NS_TYPE_NONE==0.  Also save a test-n-branch.
> 
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Glauber Costa <glommer@parallels.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Tejun Heo <tj@kernel.org>

Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>

> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  fs/sysfs/dir.c |    8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff -puN fs/sysfs/dir.c~sysfs-fail-dentry-revalidation-after-namespace-change-fix fs/sysfs/dir.c
> --- a/fs/sysfs/dir.c~sysfs-fail-dentry-revalidation-after-namespace-change-fix
> +++ a/fs/sysfs/dir.c
> @@ -329,10 +329,12 @@ static int sysfs_dentry_revalidate(struc
>  
>  	/* The sysfs dirent has been moved to a different namespace */
>  	type = KOBJ_NS_TYPE_NONE;
> -	if (sd->s_parent)
> +	if (sd->s_parent) {
>  		type = sysfs_ns_type(sd->s_parent);
> -	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
> -		goto out_bad;
> +		if (type != KOBJ_NS_TYPE_NONE &&
> +				sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns)
> +			goto out_bad;
> +	}
>  
>  	mutex_unlock(&sysfs_mutex);
>  out_valid:
> _
> 
> 

Re: [PATCH v2] fail dentry revalidation after namespace change

[Eric W. Biederman]

Andrew Morton <akpm@linux-foundation.org> writes:

>>  {
>>  	struct sysfs_dirent *sd;
>>  	int is_dir;
>> +	int type;
>>  
>>  	if (nd->flags & LOOKUP_RCU)
>>  		return -ECHILD;
>> @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>>  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
>>  		goto out_bad;
>>  
>> +	/* The sysfs dirent has been moved to a different namespace */
>> +	type = KOBJ_NS_TYPE_NONE;
>> +	if (sd->s_parent)
>> +		type = sysfs_ns_type(sd->s_parent);
>> +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
>
> eww, the code is assuming that KOBJ_NS_TYPE_NONE has a value of zero. 
> Don't do that; it smells bad.

Gag.  An incomplete change in idiom.

KOBJ_NS_TYPE_NONE is explicitly defined as 0 so that it can be used
this way, and every where else in fs/sysfs/dir.c uses this idiom.

Furthermore your change below takes one line of readable code and turns
it into something inappropriate to talk about in polite company.

If you want the code to be perfect type should be defined as
"enum kobj_ns_type type" instead of "int kobj_ns_type".

Of course the truly perfect solution is to rework the sysfs
code in a manner similar to proc, with magic internal symlinks
and multiple parallel tress for the different namespaces.
For the users of sysfs semantically there would be no changes
but in the implementation there would many fewer special cases
for namespaces.   The only special case would be reduced to
the internal sysfs symlink that lookup would have to know
about.

> @@ -329,10 +329,12 @@ static int sysfs_dentry_revalidate(struc
>  
>  	/* The sysfs dirent has been moved to a different namespace */
>  	type = KOBJ_NS_TYPE_NONE;
> -	if (sd->s_parent)
> +	if (sd->s_parent) {
>  		type = sysfs_ns_type(sd->s_parent);
> -	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
> -		goto out_bad;
> +		if (type != KOBJ_NS_TYPE_NONE &&
> +				sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns)
> +			goto out_bad;
> +	}

Pray tell in what parallel universe is that monstrosity above more
readable than the line it replaces?

Eric

Re: [PATCH v2] fail dentry revalidation after namespace change

[Andrew Morton]

On Mon, 09 Jul 2012 17:30:48 -0700 ebiederm@xmission.com (Eric W. Biederman) wrote:

> Andrew Morton <akpm@linux-foundation.org> writes:
> 
> >>  {
> >>  	struct sysfs_dirent *sd;
> >>  	int is_dir;
> >> +	int type;
> >>  
> >>  	if (nd->flags & LOOKUP_RCU)
> >>  		return -ECHILD;
> >> @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
> >>  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
> >>  		goto out_bad;
> >>  
> >> +	/* The sysfs dirent has been moved to a different namespace */
> >> +	type = KOBJ_NS_TYPE_NONE;
> >> +	if (sd->s_parent)
> >> +		type = sysfs_ns_type(sd->s_parent);
> >> +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
> >
> > eww, the code is assuming that KOBJ_NS_TYPE_NONE has a value of zero. 
> > Don't do that; it smells bad.
> 
> Gag.  An incomplete change in idiom.
> 
> KOBJ_NS_TYPE_NONE is explicitly defined as 0 so that it can be used
> this way, and every where else in fs/sysfs/dir.c uses this idiom.

One man's idiom is another man's idiocy.

Seriously.  What sort of idea is that?  Create an enumerated type and
then just ignore it?

> Pray tell in what parallel universe is that monstrosity above more
> readable than the line it replaces?

Don't be silly, it is not a "monstrosity".  The code it is modifying
contains an unneeded test-and-branch.  It's a test and branch which the
compiler might be able to avoid.  If we can demonstrate that the
compiler does indeed optimise it, or if we can find a less monstrous
way of implementing it then fine.  Otherwise, efficiency wins.

Re: [PATCH v2] fail dentry revalidation after namespace change

[Eric W. Biederman]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Mon, 09 Jul 2012 17:30:48 -0700 ebiederm@xmission.com (Eric W. Biederman) wrote:
>
>> Andrew Morton <akpm@linux-foundation.org> writes:
>> 
>> >>  {
>> >>  	struct sysfs_dirent *sd;
>> >>  	int is_dir;
>> >> +	int type;
>> >>  
>> >>  	if (nd->flags & LOOKUP_RCU)
>> >>  		return -ECHILD;
>> >> @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>> >>  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
>> >>  		goto out_bad;
>> >>  
>> >> +	/* The sysfs dirent has been moved to a different namespace */
>> >> +	type = KOBJ_NS_TYPE_NONE;
>> >> +	if (sd->s_parent)
>> >> +		type = sysfs_ns_type(sd->s_parent);
>> >> +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
>> >
>> > eww, the code is assuming that KOBJ_NS_TYPE_NONE has a value of zero. 
>> > Don't do that; it smells bad.
>> 
>> Gag.  An incomplete change in idiom.
>> 
>> KOBJ_NS_TYPE_NONE is explicitly defined as 0 so that it can be used
>> this way, and every where else in fs/sysfs/dir.c uses this idiom.
>
> One man's idiom is another man's idiocy.

And code that uses inconsistent idioms is even harder to read.

A half assed cleanup is worse than no cleanup.

> Seriously.  What sort of idea is that?  Create an enumerated type and
> then just ignore it?

It isn't ignored.  It just has a well defined NULL value. That is hardly
controversial.

>> Pray tell in what parallel universe is that monstrosity above more
>> readable than the line it replaces?
>
> Don't be silly, it is not a "monstrosity".  The code it is modifying
> contains an unneeded test-and-branch.  It's a test and branch which the
> compiler might be able to avoid.  If we can demonstrate that the
> compiler does indeed optimise it, or if we can find a less monstrous
> way of implementing it then fine.  Otherwise, efficiency wins.

Efficiency wins?  In a rarely used function?  Which kernel are you
working on?

Readable maintainable code wins.  Unreadable code causes regressions.

Your addition while it may not be monstrous is most definitely less
readable.

Eric

Re: [PATCH v2] fail dentry revalidation after namespace change

[Andrew Morton]

On Mon, 09 Jul 2012 18:51:37 -0700 ebiederm@xmission.com (Eric W. Biederman) wrote:

> Andrew Morton <akpm@linux-foundation.org> writes:
> 
> > On Mon, 09 Jul 2012 17:30:48 -0700 ebiederm@xmission.com (Eric W. Biederman) wrote:
> >
> >> Andrew Morton <akpm@linux-foundation.org> writes:
> >> 
> >> >>  {
> >> >>  	struct sysfs_dirent *sd;
> >> >>  	int is_dir;
> >> >> +	int type;
> >> >>  
> >> >>  	if (nd->flags & LOOKUP_RCU)
> >> >>  		return -ECHILD;
> >> >> @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
> >> >>  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
> >> >>  		goto out_bad;
> >> >>  
> >> >> +	/* The sysfs dirent has been moved to a different namespace */
> >> >> +	type = KOBJ_NS_TYPE_NONE;
> >> >> +	if (sd->s_parent)
> >> >> +		type = sysfs_ns_type(sd->s_parent);
> >> >> +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
> >> >
> >> > eww, the code is assuming that KOBJ_NS_TYPE_NONE has a value of zero. 
> >> > Don't do that; it smells bad.
> >> 
> >> Gag.  An incomplete change in idiom.
> >> 
> >> KOBJ_NS_TYPE_NONE is explicitly defined as 0 so that it can be used
> >> this way, and every where else in fs/sysfs/dir.c uses this idiom.
> >
> > One man's idiom is another man's idiocy.
> 
> And code that uses inconsistent idioms is even harder to read.

Not true.  That patch is more readable when it is changed to use
correct types.  If only because readers don't need to go in and check
that KOBJ_NS_TYPE_NONE has value zero.

> > Seriously.  What sort of idea is that?  Create an enumerated type and
> > then just ignore it?
> 
> It isn't ignored.  It just has a well defined NULL value. That is hardly
> controversial.

If it's uncontroversial, why are we talking about it?  Why did I, an
experienced C and kernel developer, think that it looked stupid and
possibly buggy?

I'm uncomfortable with propagating this idiotic and unnecessary trick
any further.  It's better to fix it.

> >> Pray tell in what parallel universe is that monstrosity above more
> >> readable than the line it replaces?
> >
> > Don't be silly, it is not a "monstrosity".  The code it is modifying
> > contains an unneeded test-and-branch.  It's a test and branch which the
> > compiler might be able to avoid.  If we can demonstrate that the
> > compiler does indeed optimise it, or if we can find a less monstrous
> > way of implementing it then fine.  Otherwise, efficiency wins.
> 
> Efficiency wins?  In a rarely used function?  Which kernel are you
> working on?

One in which we frequently optimise uncommon code paths.

> Readable maintainable code wins.  Unreadable code causes regressions.

Dude, the whole reason for having enums and enumerated types is for
readability and maintainability.  If we didn't care about that, we'd
use literal constants everywhere.  And here you are arguing against
that readability and maintainability.

If you want to say "yes, the sysfs code is bad but I can't be bothered
fixing it all" then grumble, but OK.  But for heavens sake, don't go
and *defend* what that code is doing.