[PATCH] tmpfs: fix mounts when size is less than the page size

[PATCH] tmpfs: fix mounts when size is less than the page size

[Michael Marineau]

When tmpfs is mounted with a size less than one page the number of blocks
is set to 0 which makes the tmpfs mount unlimited. This can lead to a quick
and surprising death is someone typos a tmpfs mount command and writes to much.

tmpfs can still be mounted as unlimited if size or nr_blocks is exactly 0.

Signed-off-by: Michael Marineau <mike@marineau.org>
---
 mm/shmem.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 253d205..66b07f2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2138,6 +2138,8 @@ static int shmem_parse_options(char *options, int *mode, uid_t *uid,
 			if (*rest)
 				goto bad_val;
 			*blocks = size >> PAGE_CACHE_SHIFT;
+			if (size && blocks == 0)
+				blocks = 1;
 		} else if (!strcmp(this_char,"nr_blocks")) {
 			*blocks = memparse(value,&rest);
 			if (*rest)
-- 
1.5.1.6

[PATCH, RESEND] tmpfs: fix mounts when size is less than the page size

[Michael Marineau]

When tmpfs is mounted with a size less than one page the number of blocks
is set to 0 which makes the tmpfs mount unlimited. This can lead to a quick
and surprising death is someone typos a tmpfs mount command and writes to much.

tmpfs can still be mounted as unlimited if size or nr_blocks is exactly 0.

Signed-off-by: Michael Marineau <mike@marineau.org>
---
Somehow I was a moron and didn't notice the compiler shouting insults at me,
here is the correct version of the patch. Sorry for the noise :-(

 mm/shmem.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 253d205..86b47d8 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2138,6 +2138,8 @@ static int shmem_parse_options(char *options, int *mode, uid_t *uid,
 			if (*rest)
 				goto bad_val;
 			*blocks = size >> PAGE_CACHE_SHIFT;
+			if (size && *blocks == 0)
+				*blocks = 1;
 		} else if (!strcmp(this_char,"nr_blocks")) {
 			*blocks = memparse(value,&rest);
 			if (*rest)
-- 
1.5.1.6

Re: [PATCH, RESEND] tmpfs: fix mounts when size is less than the page size

[Hugh Dickins]

Fair enough, that does match the documentation of "size=0" better.
Though either way, someone who typos is going to get one kind of surprise
or another.  Do you mind if we do it slightly differently, achieving the
same effect without a special case, by rounding up instead of down?


From: Michael Marineau <mike@marineau.org>

When tmpfs is mounted with a size less than one page, the number of blocks
is set to 0 which makes the tmpfs mount unlimited. This can lead to a quick
and surprising death if someone typos a tmpfs mount command and writes too
much.

tmpfs can still be mounted as unlimited if size or nr_blocks is exactly 0,
as Documentation/filesystems/tmpfs.txt says.

Hugh: do this by rounding size up instead of down in all cases: which
slightly expands other odd-sized tmpfs mounts, but in a consistent way.

Signed-off-by: Michael Marineau <mike@marineau.org>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
---

 mm/shmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 2.6.24-rc1-git/mm/shmem.c	2007-10-31 06:18:05.000000000 +0000
+++ linux/mm/shmem.c	2007-11-04 11:46:09.000000000 +0000
@@ -2137,7 +2137,7 @@ static int shmem_parse_options(char *opt
 			}
 			if (*rest)
 				goto bad_val;
-			*blocks = size >> PAGE_CACHE_SHIFT;
+			*blocks = DIV_ROUND_UP(size, PAGE_CACHE_SIZE);
 		} else if (!strcmp(this_char,"nr_blocks")) {
 			*blocks = memparse(value,&rest);
 			if (*rest)

Re: [PATCH, RESEND] tmpfs: fix mounts when size is less than the page size

[Michael Marineau]

On 11/4/07, Hugh Dickins <hugh@veritas.com> wrote:
> Fair enough, that does match the documentation of "size=0" better.
> Though either way, someone who typos is going to get one kind of surprise
> or another.  Do you mind if we do it slightly differently, achieving the
> same effect without a special case, by rounding up instead of down?

Sounds reasonable to me. I didn't know about DIV_ROUND_UP :-)

>
>
> From: Michael Marineau <mike@marineau.org>
>
> When tmpfs is mounted with a size less than one page, the number of blocks
> is set to 0 which makes the tmpfs mount unlimited. This can lead to a quick
> and surprising death if someone typos a tmpfs mount command and writes too
> much.
>
> tmpfs can still be mounted as unlimited if size or nr_blocks is exactly 0,
> as Documentation/filesystems/tmpfs.txt says.
>
> Hugh: do this by rounding size up instead of down in all cases: which
> slightly expands other odd-sized tmpfs mounts, but in a consistent way.
>
> Signed-off-by: Michael Marineau <mike@marineau.org>
> Signed-off-by: Hugh Dickins <hugh@veritas.com>
> ---
>
>  mm/shmem.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- 2.6.24-rc1-git/mm/shmem.c   2007-10-31 06:18:05.000000000 +0000
> +++ linux/mm/shmem.c    2007-11-04 11:46:09.000000000 +0000
> @@ -2137,7 +2137,7 @@ static int shmem_parse_options(char *opt
>                         }
>                         if (*rest)
>                                 goto bad_val;
> -                       *blocks = size >> PAGE_CACHE_SHIFT;
> +                       *blocks = DIV_ROUND_UP(size, PAGE_CACHE_SIZE);
>                 } else if (!strcmp(this_char,"nr_blocks")) {
>                         *blocks = memparse(value,&rest);
>                         if (*rest)
>


-- 
Michael Marineau
Oregon State University
mike@marineau.org

Re: [PATCH, RESEND] tmpfs: fix mounts when size is less than the page size

[Hugh Dickins]

On Sun, 4 Nov 2007, Michael Marineau wrote:
> On 11/4/07, Hugh Dickins <hugh@veritas.com> wrote:
> > Fair enough, that does match the documentation of "size=0" better.
> > Though either way, someone who typos is going to get one kind of surprise
> > or another.  Do you mind if we do it slightly differently, achieving the
> > same effect without a special case, by rounding up instead of down?
> 
> Sounds reasonable to me. I didn't know about DIV_ROUND_UP :-)

I'm not familiar with it either (had to look it up to check it's
the right thing) - in mm/ we're accustomed to writing things like
"(size + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT"; but that took
the line over 80 columns, and I dreaded someone somewhere mailing
in to ask "Why didn't you use DIV_ROUND_UP?" ;-)

Hugh