linux-kernel - Re: [PATCH] zram: check compressor name before setting it

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Date:	Mon, 25 May 2015 15:18:38 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Marcin Jabrzyk <m.jabrzyk@...sung.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	minchan@...nel.org, ngupta@...are.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kyungmin.park@...sung.com
Subject: Re: [PATCH] zram: check compressor name before setting it

On (05/22/15 15:26), Marcin Jabrzyk wrote:
> >> From the other hand, the only valid values that can be written are
> >>in 'comp_algorithm'.
> >>So when writing other one, returning -EINVAL seems to be reasonable.
> >>The user would get immediately information that he can't do that,
> >>now the information can be very deferred in time.
> >
> >it's not.
> >the error message appears in syslog right before we return -EINVAL
> >back to user.
> 
> Yes I've read up the code more detailed and I saw that error message
> just before returning to user with error value.
> 
> But this happens when 'disksize' is wirtten, not when 'comp_algorithm'.
> I understood, the error message in dmesg is clear there is no such
> algorithm.
> 
> But this is not an immediate error, when setting the 'comp_algorithm',
> where we already know that it's wrong, not existing etc.
> Anything after that moment would be wrong and would not work at all.
> 
> From what I saw 'comp_algorithm_store' is the only *_store in zram that
> believes user that he writes proper value and just makes strlcpy.
> 
> So what I've ing mind is to provide direct feedback, you have
> written wrong name of compressor, you got -EINVAL, please write
> correct value. This would be very useful when scripting.
> 

I can't see how printing error 0.0012 seconds earlier helps. really.
if one sets a compression algorithm the very next thing to do is to
set device's disksize. even if he/she usually watch a baseball game in
between, then the error message appears right when it's needed anyway:
during `setup my device and make it usable' stage.


>I'm not for exposing more internals, but getting -EINVAL would be nice I

you are.

find_backend() returns back to its caller a raw and completely initialized
zcomp_backend pointer. this is very dangerous zcomp internals, which should
never be exposed. from zcomp layer we return either ERR_PTR() or a usable
zcomp_backend pointer. that's the rule.


if you guys still insist that this is critical and very important change,
then there should be a small helper function instead with a clear name
(starting with zcomp_ to indicate its place) which will simply return bool
TRUE for `algorithm is known' case and FALSE otherwise (aka `algorithm is
unknown').


something like below:


  # echo LZ5 > /sys/block/zram0/comp_algorithm
  -bash: echo: write error: Invalid argument

  dmesg
  [ 7440.544852] Error: unknown compression algorithm: LZ5


p.s. but, I still see a very little value.
p.p.s notice a necessary and inevitable `dmesg'. (back to 'no one reads
syslog' argument).

---

 drivers/block/zram/zcomp.c    | 10 ++++++++++
 drivers/block/zram/zcomp.h    |  1 +
 drivers/block/zram/zram_drv.c |  3 +++
 3 files changed, 14 insertions(+)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index a1a8b8e..b68b16f 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -54,11 +54,16 @@ static struct zcomp_backend *backends[] = {
 static struct zcomp_backend *find_backend(const char *compress)
 {
 	int i = 0;
+
 	while (backends[i]) {
 		if (sysfs_streq(compress, backends[i]->name))
 			break;
 		i++;
 	}
+
+	if (!backends[i])
+		pr_err("Error: unknown compression algorithm: %s\n",
+				compress);
 	return backends[i];
 }
 
@@ -320,6 +325,11 @@ void zcomp_destroy(struct zcomp *comp)
 	kfree(comp);
 }
 
+bool zcomp_known_algorithm(const char *comp)
+{
+	return find_backend(comp) != NULL;
+}
+
 /*
  * search available compressors for requested algorithm.
  * allocate new zcomp and initialize it. return compressing
diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
index c59d1fc..773bdf1 100644
--- a/drivers/block/zram/zcomp.h
+++ b/drivers/block/zram/zcomp.h
@@ -51,6 +51,7 @@ struct zcomp {
 };
 
 ssize_t zcomp_available_show(const char *comp, char *buf);
+bool zcomp_known_algorithm(const char *comp);
 
 struct zcomp *zcomp_create(const char *comp, int max_strm);
 void zcomp_destroy(struct zcomp *comp);
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index f750e34..2197a81 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -378,6 +378,9 @@ static ssize_t comp_algorithm_store(struct device *dev,
 	if (sz > 0 && zram->compressor[sz - 1] == '\n')
 		zram->compressor[sz - 1] = 0x00;
 
+	if (!zcomp_known_algorithm(zram->compressor))
+		len = -EINVAL;
+
 	up_write(&zram->init_lock);
 	return len;
 }
--
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