LinuxLists.cc - [PATCH] init/do_mounts.c: replace sys_mount() to do

2010-08-17 12:37:30

Subject: [PATCH] init/do_mounts.c: replace sys_mount() to do_mount()

2010-08-17 13:15:31

Subject: Re: [PATCH] init/do_mounts.c: replace sys_mount() to do_mount()

On Tuesday 17 August 2010, Namhyung Kim wrote:
> static int __init do_mount_root(char *name, char *fs, int flags, void *data)
> {
> - int err = sys_mount(name, "/root", fs, flags, data);
> + int err;
> + unsigned long data_page = 0;
> +
> + if (data) {
> + /* data should be in a page */
> + data_page = __get_free_page(GFP_KERNEL);
> + if (!data_page)
> + return -ENOMEM;
> + strlcpy((char *) data_page, data, PAGE_SIZE);
> + }
> +
> + err = do_mount(name, "/root", fs, flags, (void *) data_page);
> +
> + if (data_page)
> + free_page(data_page);

AFAICT there is no need for the copy at all here. The reason that sys_mount
copies to a separate page is that it gets passed an arbitrary-length user
input.

Arnd

2010-08-17 13:25:28

by Namhyung Kim

[permalink] [raw]

Subject: Re: [PATCH] init/do_mounts.c: replace sys_mount() to do_mount()

2010-08-17 (화), 15:15 +0200, Arnd Bergmann:
> AFAICT there is no need for the copy at all here. The reason that sys_mount
> copies to a separate page is that it gets passed an arbitrary-length user
> input.
>
> Arnd

Right. But make sure to user options never exceed a page, do_mount() has
following code:

if (data_page)
((char *)data_page)[PAGE_SIZE - 1] = 0;

--
Regards,
Namhyung Kim

2010-08-17 13:42:32

by Arnd Bergmann

[permalink] [raw]

Subject: Re: [PATCH] init/do_mounts.c: replace sys_mount() to do_mount()

On Tuesday 17 August 2010, Namhyung Kim wrote:
> 2010-08-17 (화), 15:15 +0200, Arnd Bergmann:
> > AFAICT there is no need for the copy at all here. The reason that sys_mount
> > copies to a separate page is that it gets passed an arbitrary-length user
> > input.
>
> Right. But make sure to user options never exceed a page, do_mount() has
> following code:
>
> if (data_page)
> ((char *)data_page)[PAGE_SIZE - 1] = 0;

Hmm, how very unexpected...

Maybe you can fix that as well in the same patch and move this safeguard
into the copy_mount_options() function? I guess ideally we'd even mark
the arguments to do_mount() as pointers to const, but that might create
an excessive amount of churn.

Arnd

2010-08-17 14:38:22

by Namhyung Kim

[permalink] [raw]

Subject: [PATCH v2] init: replace sys_mount() to do_mount()

sys_mount() just copies all (string) arguments from user space to kernel
and calls do_mount(). In this case we have all args in kernel already so
there is no need to call sys_mount(). One thing we should take care is
'data' have to be in a page unless it is NULL. Do it manually.

Signed-off-by: Namhyung Kim <[email protected]>
---

I found 3 more calls in init/do_mounts_initrd.c and merged them. Please
apply this instead.

2010-08-17 (Tue), 15:41 +0200, Arnd Bergmann:
> Maybe you can fix that as well in the same patch and move this safeguard
> into the copy_mount_options() function? I guess ideally we'd even mark
> the arguments to do_mount() as pointers to const, but that might create
> an excessive amount of churn.

Arnd, thanks for reviewing and the suggestion but I think that should be
in a different patch (set).

init/do_mounts.c | 18 ++++++++++++++++--
init/do_mounts_initrd.c | 6 +++---
2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 02e3ca4..8eabff6 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -217,7 +217,21 @@ static void __init get_fs_names(char *page)

static int __init do_mount_root(char *name, char *fs, int flags, void *data)
{
- int err = sys_mount(name, "/root", fs, flags, data);
+ int err;
+ unsigned long data_page = 0;
+
+ if (data) {
+ /* data should be in a page */
+ data_page = __get_free_page(GFP_KERNEL);
+ if (!data_page)
+ return -ENOMEM;
+ strlcpy((char *) data_page, data, PAGE_SIZE);
+ }
+
+ err = do_mount(name, "/root", fs, flags, (void *) data_page);
+
+ if (data_page)
+ free_page(data_page);
if (err)
return err;

@@ -417,6 +431,6 @@ void __init prepare_namespace(void)
mount_root();
out:
devtmpfs_mount("dev");
- sys_mount(".", "/", NULL, MS_MOVE, NULL);
+ do_mount(".", "/", NULL, MS_MOVE, NULL);
sys_chroot(".");
}
diff --git a/init/do_mounts_initrd.c b/init/do_mounts_initrd.c
index 2b10853..8355501 100644
--- a/init/do_mounts_initrd.c
+++ b/init/do_mounts_initrd.c
@@ -48,7 +48,7 @@ static void __init handle_initrd(void)
old_fd = sys_open("/old", 0, 0);
/* move initrd over / and chdir/chroot in initrd root */
sys_chdir("/root");
- sys_mount(".", "/", NULL, MS_MOVE, NULL);
+ do_mount(".", "/", NULL, MS_MOVE, NULL);
sys_chroot(".");

/*
@@ -66,7 +66,7 @@ static void __init handle_initrd(void)

/* move initrd to rootfs' /old */
sys_fchdir(old_fd);
- sys_mount("/", ".", NULL, MS_MOVE, NULL);
+ do_mount("/", ".", NULL, MS_MOVE, NULL);
/* switch root and cwd back to / of rootfs */
sys_fchdir(root_fd);
sys_chroot(".");
@@ -82,7 +82,7 @@ static void __init handle_initrd(void)
mount_root();

printk(KERN_NOTICE "Trying to move old root to /initrd ... ");
- error = sys_mount("/old", "/root/initrd", NULL, MS_MOVE, NULL);
+ error = do_mount("/old", "/root/initrd", NULL, MS_MOVE, NULL);
if (!error)
printk("okay\n");
else {
--
1.7.0.4