On Thu, 20 Sep 2007 12:52:56 -0700 Dave Hansen <[email protected]> wrote:
> This is the first really tricky patch in the series. It
> elevates the writer count on a mount each time a
> non-special file is opened for write.
>
> This is not completely apparent in the patch because the
> two if() conditions in may_open() above the
> mnt_want_write() call are, combined, equivalent to
> special_file().
>
> There is also an elevated count around the vfs_create()
> call in open_namei(). The count needs to be kept elevated
> all the way into the may_open() call. Otherwise, when the
> write is dropped, a ro->rw transisition could occur. This
> would lead to having rw access on the newly created file,
> while the vfsmount is ro. That is bad.
>
> Some filesystems forego the use of normal vfs calls to create
> struct files. Make sure that these users elevate the mnt writer
> count because they will get __fput(), and we need to make
> sure they're balanced.
For some reason this has started oopsing on me:
[ 39.941273] audit(1196237507.111:5): audit_pid=1882 old=0 by auid=4294967295 subj=system_u:system_r:auditd_t:s0
[ 35.037235] BUG: unable to handle kernel NULL pointer dereference at virtual address 0000006a
[ 35.040536] printing eip: c017815c *pde = 00000000
[ 35.043913] Oops: 0000 [#1] PREEMPT
[ 35.047270] last sysfs file: /devices/platform/i8042/serio0/description
[ 35.050679] Modules linked in: ipv6 autofs4 hidp l2cap bluetooth sunrpc nf_conntrack_netbios_ns ipt_REJECT nf_conntrack_ipv4 xt_state nf_conntrack xt_tcpudp iptable_filter ip_tables x_tables acpi_cpufreq nvram ohci1394 ieee1394 ehci_hcd uhci_hcd sg joydev snd_hda_intel snd_seq_dummy snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device snd_pcm_oss snd_mixer_oss snd_pcm sr_mod snd_timer snd i2c_i801 ipw2200 cdrom ieee80211 pcspkr piix soundcore ieee80211_crypt i2c_core snd_page_alloc button generic ext3 jbd ide_disk ide_core
[ 35.071690]
[ 35.075612] Pid: 2277, comm: hald-addon-acpi Not tainted (2.6.24-rc3-mm2 #8)
[ 35.080517] EIP: 0060:[<c017815c>] EFLAGS: 00010296 CPU: 0
[ 35.084585] EIP is at open_pathname+0x37b/0x6a5
[ 35.088631] EAX: 00000000 EBX: fffffff0 ECX: f61fc000 EDX: c01a02c1
[ 35.092747] ESI: f61fdf20 EDI: 00000008 EBP: f61fdf84 ESP: f61fdefc
[ 35.098762] DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 0068
[ 35.102847] Process hald-addon-acpi (pid: 2277, ti=F61FC000 task=F61FEEB0 task.ti=F61FC000)
[ 35.103028] Stack: 00008000 f6589000 00000000 00000004 00000000 00008001 00000000 00000246
[ 35.107359] 0000023a f5a9f908 f780cbc0 00000000 00000000 00000000 00000101 00000001
[ 35.111671] 00000000 f61fdf5c 00000246 c016cdd9 00000246 f782b6a4 00000004 f782b6a4
[ 35.115978] Call Trace:
[ 35.124180] [<c0104a74>] show_trace_log_lvl+0x12/0x25
[ 35.128422] [<c0104b13>] show_stack_log_lvl+0x8c/0x97
[ 35.132646] [<c0104ba8>] show_registers+0x8a/0x1c0
[ 35.136900] [<c0104dcc>] die+0xee/0x1c4
[ 35.141165] [<c011683c>] do_page_fault+0x405/0x4e1
[ 35.145486] [<c031db7a>] error_code+0x6a/0x70
[ 35.149672] [<c016e02b>] do_sys_open+0x42/0xb8
[ 35.153714] [<c016e0cd>] sys_open+0x16/0x18
[ 35.157763] [<c0103b2a>] syscall_call+0x7/0xb
[ 35.161831] =======================
[ 35.165899] Code: 7d 80 00 0f 84 a9 00 00 00 8b 45 a0 e8 9e 94 00 00 e9 9c 00 00 00 8b 95 78 ff ff ff 89 f0 e8 23 4f ff ff 89 c3 8b 45 9c 8b 40 28 <0f> b7 40 6a 25 00 f0 00 00 3d 00 20 00 00 0f 84 0c 03 00 00 3d
[ 35.175160] EIP: [<c017815c>] open_pathname+0x37b/0x6a5 SS:ESP 0068:f61fdefc
I debugged it a bit. hald-addon-acpi is opening /proc/acpi/event and we're
getting here:
error = may_open(&nd, acc_mode, flag);
if (error) {
if (open_will_write_to_fs(flag, nd.dentry->d_inode))
mnt_drop_write(nd.mnt);
goto exit;
}
filp = nameidata_to_filp(&nd, sys_open_flag);
/*
* It is now safe to drop the mnt write
* because the filp has had a write taken
* on its behalf.
*/
if (open_will_write_to_fs(flag, nd.dentry->d_inode))
mnt_drop_write(nd.mnt);
return filp;
the final call to open_will_write_to_fs() is oopsing over null
nd.dentry->d_inode.
Given that nameidata_to_filp() can call path_release() which "destroys the
original nameidata", it looks like this was always buggy?
This:
--- a/fs/namei.c~b
+++ a/fs/namei.c
@@ -1722,7 +1722,7 @@ static inline int sys_open_flags_to_name
return flag;
}
-static inline int open_will_write_to_fs(int flag, struct inode *inode)
+static int open_will_write_to_fs(int flag, struct inode *inode)
{
/*
* We'll never write to the fs underlying
@@ -1752,6 +1752,7 @@ struct file *open_pathname(int dfd, cons
struct path path;
struct dentry *dir;
int count = 0;
+ int will_write;
int flag = sys_open_flags_to_namei_flags(sys_open_flag);
acc_mode = ACC_MODE(flag);
@@ -1870,14 +1871,15 @@ ok:
* be avoided. Taking this mnt write here
* ensures that (2) can not occur.
*/
- if (open_will_write_to_fs(flag, nd.dentry->d_inode)) {
+ will_write = open_will_write_to_fs(flag, nd.dentry->d_inode);
+ if (will_write) {
error = mnt_want_write(nd.mnt);
if (error)
goto exit;
}
error = may_open(&nd, acc_mode, flag);
if (error) {
- if (open_will_write_to_fs(flag, nd.dentry->d_inode))
+ if (will_write)
mnt_drop_write(nd.mnt);
goto exit;
}
@@ -1887,7 +1889,7 @@ ok:
* because the filp has had a write taken
* on its behalf.
*/
- if (open_will_write_to_fs(flag, nd.dentry->d_inode))
+ if (will_write)
mnt_drop_write(nd.mnt);
return filp;
_
seems a fairly obvious fix and is more efficient anyway.
I hate this patch series.
-
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