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Subject[PATCH v2 23/35] ovl: copy-up on MAP_SHARED
Date
A corner case of a corner case is when

- file opened for O_RDONLY
- which is then memory mapped SHARED
- file opened for O_WRONLY
- contents modified
- contents read back though the shared mapping

Unfortunately it looks very difficult to do anything about the established
shared map after the file is copied up.

Instead, when a read-only file is mapped shared, copy up the file before
actually doing the map. This may result in unnecessary copy-ups (but so
may copy-up on open(O_RDWR) for exampe).

We can revisit this later if it turns out to be a performance problem in
real life.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
fs/overlayfs/Kconfig | 21 +++++++++++++++++++++
fs/overlayfs/file.c | 22 ++++++++++++++++++++++
fs/overlayfs/overlayfs.h | 7 +++++++
fs/overlayfs/ovl_entry.h | 1 +
fs/overlayfs/super.c | 22 ++++++++++++++++++++++
5 files changed, 73 insertions(+)

diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
index 17032631c5cf..991c0a5a0e00 100644
--- a/fs/overlayfs/Kconfig
+++ b/fs/overlayfs/Kconfig
@@ -103,3 +103,24 @@ config OVERLAY_FS_XINO_AUTO
For more information, see Documentation/filesystems/overlayfs.txt

If unsure, say N.
+
+config OVERLAY_FS_COPY_UP_SHARED
+ bool "Overlayfs: copy up when mapping a file shared"
+ default n
+ depends on OVERLAY_FS
+ help
+ If this option is enabled then on mapping a file with MAP_SHARED
+ overlayfs copies up the file in anticipation of it being modified (just
+ like we copy up the file on O_WRONLY and O_RDWR in anticipation of
+ modification). This does not interfere with shared library loading, as
+ that uses MAP_PRIVATE. But there might be use cases out there where
+ this impacts performance and disk usage.
+
+ This just selects the default, the feature can also be enabled or
+ disabled in the running kernel or individually on each overlay mount.
+
+ To get maximally standard compliant behavior, enable this option.
+
+ To get a maximally backward compatible kernel, disable this option.
+
+ If unsure, say N.
diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 2ac95c95e8e6..a60734ec89ec 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -10,6 +10,7 @@
#include <linux/file.h>
#include <linux/mount.h>
#include <linux/xattr.h>
+#include <linux/mman.h>
#include <linux/uio.h>
#include "overlayfs.h"

@@ -256,6 +257,26 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
return ret;
}

+static int ovl_pre_mmap(struct file *file, unsigned long prot,
+ unsigned long flag)
+{
+ int err = 0;
+
+ /*
+ * Take MAP_SHARED as hint about future writes to the file (through
+ * another file descriptor). Caller might not have had such an intent,
+ * but we hope MAP_PRIVATE will be used in most such cases.
+ *
+ * If we don't copy up now and the file is modified, it becomes really
+ * difficult to change the mapping to match that of the file's content
+ * later.
+ */
+ if ((flag & MAP_SHARED) && ovl_copy_up_shared(file_inode(file)->i_sb))
+ err = ovl_copy_up(file_dentry(file));
+
+ return err;
+}
+
static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
{
struct fd real;
@@ -473,6 +494,7 @@ const struct file_operations ovl_file_operations = {
.read_iter = ovl_read_iter,
.write_iter = ovl_write_iter,
.fsync = ovl_fsync,
+ .pre_mmap = ovl_pre_mmap,
.mmap = ovl_mmap,
.fallocate = ovl_fallocate,
.unlocked_ioctl = ovl_ioctl,
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index edbf69c8f45d..caaa47cea2aa 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -272,6 +272,13 @@ static inline unsigned int ovl_xino_bits(struct super_block *sb)
return ofs->xino_bits;
}

+static inline bool ovl_copy_up_shared(struct super_block *sb)
+{
+ struct ovl_fs *ofs = sb->s_fs_info;
+
+ return !(sb->s_flags & SB_RDONLY) && ofs->config.copy_up_shared;
+}
+

/* namei.c */
int ovl_check_fh_len(struct ovl_fh *fh, int fh_len);
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index 41655a7d6894..3bea47c63fd9 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -18,6 +18,7 @@ struct ovl_config {
const char *redirect_mode;
bool index;
bool nfs_export;
+ bool copy_up_shared;
int xino;
};

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index ad6a5baf226b..c3d8c7ea180f 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -56,6 +56,12 @@ module_param_named(xino_auto, ovl_xino_auto_def, bool, 0644);
MODULE_PARM_DESC(ovl_xino_auto_def,
"Auto enable xino feature");

+static bool ovl_copy_up_shared_def =
+ IS_ENABLED(CONFIG_OVERLAY_FS_COPY_UP_SHARED);
+module_param_named(copy_up_shared, ovl_copy_up_shared_def, bool, 0644);
+MODULE_PARM_DESC(ovl_copy_up_shared_def,
+ "Copy up when mapping a file shared");
+
static void ovl_entry_stack_free(struct ovl_entry *oe)
{
unsigned int i;
@@ -380,6 +386,9 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
"on" : "off");
if (ofs->config.xino != ovl_xino_def())
seq_printf(m, ",xino=%s", ovl_xino_str[ofs->config.xino]);
+ if (ofs->config.copy_up_shared != ovl_copy_up_shared_def)
+ seq_printf(m, ",copy_up_shared=%s",
+ ofs->config.copy_up_shared ? "on" : "off");
return 0;
}

@@ -417,6 +426,8 @@ enum {
OPT_XINO_ON,
OPT_XINO_OFF,
OPT_XINO_AUTO,
+ OPT_COPY_UP_SHARED_ON,
+ OPT_COPY_UP_SHARED_OFF,
OPT_ERR,
};

@@ -433,6 +444,8 @@ static const match_table_t ovl_tokens = {
{OPT_XINO_ON, "xino=on"},
{OPT_XINO_OFF, "xino=off"},
{OPT_XINO_AUTO, "xino=auto"},
+ {OPT_COPY_UP_SHARED_ON, "copy_up_shared=on"},
+ {OPT_COPY_UP_SHARED_OFF, "copy_up_shared=off"},
{OPT_ERR, NULL}
};

@@ -559,6 +572,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
config->xino = OVL_XINO_AUTO;
break;

+ case OPT_COPY_UP_SHARED_ON:
+ config->copy_up_shared = true;
+ break;
+
+ case OPT_COPY_UP_SHARED_OFF:
+ config->copy_up_shared = false;
+ break;
+
default:
pr_err("overlayfs: unrecognized mount option \"%s\" or missing value\n", p);
return -EINVAL;
@@ -1380,6 +1401,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
ofs->config.index = ovl_index_def;
ofs->config.nfs_export = ovl_nfs_export_def;
ofs->config.xino = ovl_xino_def();
+ ofs->config.copy_up_shared = ovl_copy_up_shared_def;
err = ovl_parse_opt((char *) data, &ofs->config);
if (err)
goto out_err;
--
2.14.3
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