RE: [PATCH 09/14] d_path: introduce struct prepend_buffer

Justin He
2021-06-23T13:29:16Z

Hi Al

> -----Original Message-----
> From: Al Viro <viro@ftp.linux.org.uk> On Behalf Of Al Viro
> Sent: Wednesday, May 19, 2021 8:49 AM
> To: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Justin He <Justin.He@arm.com>; Petr Mladek <pmladek@suse.com>; Steven
> Rostedt <rostedt@goodmis.org>; Sergey Senozhatsky
> <senozhatsky@chromium.org>; Andy Shevchenko
> <andriy.shevchenko@linux.intel.com>; Rasmus Villemoes
> <linux@rasmusvillemoes.dk>; Jonathan Corbet <corbet@lwn.net>; Heiko
> Carstens <hca@linux.ibm.com>; Vasily Gorbik <gor@linux.ibm.com>; Christian
> Borntraeger <borntraeger@de.ibm.com>; Eric W . Biederman
> <ebiederm@xmission.com>; Darrick J. Wong <darrick.wong@oracle.com>; Peter
> Zijlstra (Intel) <peterz@infradead.org>; Ira Weiny <ira.weiny@intel.com>;
> Eric Biggers <ebiggers@google.com>; Ahmed S. Darwish
> <a.darwish@linutronix.de>; open list:DOCUMENTATION <linux-
> doc@vger.kernel.org>; Linux Kernel Mailing List <linux-
> kernel@vger.kernel.org>; linux-s390 <linux-s390@vger.kernel.org>; linux-
> fsdevel <linux-fsdevel@vger.kernel.org>
> Subject: [PATCH 09/14] d_path: introduce struct prepend_buffer
>
> We've a lot of places where we have pairs of form (pointer to end
> of buffer, amount of space left in front of that). These sit in pairs of
> variables located next to each other and usually passed by reference.
> Turn those into instances of new type (struct prepend_buffer) and pass
> reference to the pair instead of pairs of references to its fields.
>
> Declared and initialized by DECLARE_BUFFER(name, buf, buflen).
>
> extract_string(prepend_buffer) returns the buffer contents if
> no overflow has happened, ERR_PTR(ENAMETOOLONG) otherwise.
> All places where we used to have that boilerplate converted to use
> of that helper.
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
> fs/d_path.c | 142 ++++++++++++++++++++++++++++++++-----------------------
> -----
> 1 file changed, 75 insertions(+), 67 deletions(-)
>
> diff --git a/fs/d_path.c b/fs/d_path.c
> index 83db83446afd..06e93dd031bf 100644
> --- a/fs/d_path.c
> +++ b/fs/d_path.c
> @@ -8,12 +8,26 @@
> #include <linux/prefetch.h>
> #include "mount.h"
>
> -static void prepend(char **buffer, int *buflen, const char *str, int
> namelen)
> +struct prepend_buffer {
> + char *buf;
> + int len;
> +};
> +#define DECLARE_BUFFER(__name, __buf, __len) \
> + struct prepend_buffer __name = {.buf = __buf + __len, .len = __len}
> +
> +static char *extract_string(struct prepend_buffer *p)
> {
> - *buflen -= namelen;
> - if (likely(*buflen >= 0)) {
> - *buffer -= namelen;
> - memcpy(*buffer, str, namelen);
> + if (likely(p->len >= 0))
> + return p->buf;
> + return ERR_PTR(-ENAMETOOLONG);
> +}
> +
> +static void prepend(struct prepend_buffer *p, const char *str, int
> namelen)
> +{
> + p->len -= namelen;
> + if (likely(p->len >= 0)) {
> + p->buf -= namelen;
> + memcpy(p->buf, str, namelen);
> }
> }
>
> @@ -34,22 +48,22 @@ static void prepend(char **buffer, int *buflen, const
> char *str, int namelen)
> *
> * Load acquire is needed to make sure that we see that terminating NUL.
> */
> -static bool prepend_name(char **buffer, int *buflen, const struct qstr
> *name)
> +static bool prepend_name(struct prepend_buffer *p, const struct qstr
> *name)

Please also change the parameter description in the comments of
prepend_name(), otherwise "make C=1 W=1" will report warnings.

--
Cheers,
Justin (Jia He)

> {
> const char *dname = smp_load_acquire(&name->name); /* ^^^ */
> u32 dlen = READ_ONCE(name->len);
> - char *p;
> + char *s;
>
> - *buflen -= dlen + 1;
> - if (unlikely(*buflen < 0))
> + p->len -= dlen + 1;
> + if (unlikely(p->len < 0))
> return false;
> - p = *buffer -= dlen + 1;
> - *p++ = '/';
> + s = p->buf -= dlen + 1;
> + *s++ = '/';
> while (dlen--) {
> char c = *dname++;
> if (!c)
> break;
> - *p++ = c;
> + *s++ = c;
> }
> return true;
> }
> @@ -73,15 +87,14 @@ static bool prepend_name(char **buffer, int *buflen,
> const struct qstr *name)
> */
> static int prepend_path(const struct path *path,
> const struct path *root,
> - char **buffer, int *buflen)
> + struct prepend_buffer *p)
> {
> struct dentry *dentry;
> struct vfsmount *vfsmnt;
> struct mount *mnt;
> int error = 0;
> unsigned seq, m_seq = 0;
> - char *bptr;
> - int blen;
> + struct prepend_buffer b;
>
> rcu_read_lock();
> restart_mnt:
> @@ -89,8 +102,7 @@ static int prepend_path(const struct path *path,
> seq = 0;
> rcu_read_lock();
> restart:
> - bptr = *buffer;
> - blen = *buflen;
> + b = *p;
> error = 0;
> dentry = path->dentry;
> vfsmnt = path->mnt;
> @@ -105,8 +117,7 @@ static int prepend_path(const struct path *path,
>
> /* Escaped? */
> if (dentry != vfsmnt->mnt_root) {
> - bptr = *buffer;
> - blen = *buflen;
> + b = *p;
> error = 3;
> break;
> }
> @@ -127,7 +138,7 @@ static int prepend_path(const struct path *path,
> }
> parent = dentry->d_parent;
> prefetch(parent);
> - if (!prepend_name(&bptr, &blen, &dentry->d_name))
> + if (!prepend_name(&b, &dentry->d_name))
> break;
>
> dentry = parent;
> @@ -148,11 +159,10 @@ static int prepend_path(const struct path *path,
> }
> done_seqretry(&mount_lock, m_seq);
>
> - if (blen == *buflen)
> - prepend(&bptr, &blen, "/", 1);
> + if (b.len == p->len)
> + prepend(&b, "/", 1);
>
> - *buffer = bptr;
> - *buflen = blen;
> + *p = b;
> return error;
> }
>
> @@ -176,24 +186,24 @@ char *__d_path(const struct path *path,
> const struct path *root,
> char *buf, int buflen)
> {
> - char *res = buf + buflen;
> + DECLARE_BUFFER(b, buf, buflen);
>
> - prepend(&res, &buflen, "", 1);
> - if (prepend_path(path, root, &res, &buflen) > 0)
> + prepend(&b, "", 1);
> + if (prepend_path(path, root, &b) > 0)
> return NULL;
> - return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
> + return extract_string(&b);
> }
>
> char *d_absolute_path(const struct path *path,
> char *buf, int buflen)
> {
> struct path root = {};
> - char *res = buf + buflen;
> + DECLARE_BUFFER(b, buf, buflen);
>
> - prepend(&res, &buflen, "", 1);
> - if (prepend_path(path, &root, &res, &buflen) > 1)
> + prepend(&b, "", 1);
> + if (prepend_path(path, &root, &b) > 1)
> return ERR_PTR(-EINVAL);
> - return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
> + return extract_string(&b);
> }
>
> static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
> @@ -224,7 +234,7 @@ static void get_fs_root_rcu(struct fs_struct *fs,
> struct path *root)
> */
> char *d_path(const struct path *path, char *buf, int buflen)
> {
> - char *res = buf + buflen;
> + DECLARE_BUFFER(b, buf, buflen);
> struct path root;
>
> /*
> @@ -245,13 +255,13 @@ char *d_path(const struct path *path, char *buf, int
> buflen)
> rcu_read_lock();
> get_fs_root_rcu(current->fs, &root);
> if (unlikely(d_unlinked(path->dentry)))
> - prepend(&res, &buflen, " (deleted)", 11);
> + prepend(&b, " (deleted)", 11);
> else
> - prepend(&res, &buflen, "", 1);
> - prepend_path(path, &root, &res, &buflen);
> + prepend(&b, "", 1);
> + prepend_path(path, &root, &b);
> rcu_read_unlock();
>
> - return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
> + return extract_string(&b);
> }
> EXPORT_SYMBOL(d_path);
>
> @@ -278,36 +288,34 @@ char *dynamic_dname(struct dentry *dentry, char
> *buffer, int buflen,
>
> char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
> {
> - char *end = buffer + buflen;
> + DECLARE_BUFFER(b, buffer, buflen);
> /* these dentries are never renamed, so d_lock is not needed */
> - prepend(&end, &buflen, " (deleted)", 11);
> - prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len);
> - prepend(&end, &buflen, "/", 1);
> - return buflen >= 0 ? end : ERR_PTR(-ENAMETOOLONG);
> + prepend(&b, " (deleted)", 11);
> + prepend(&b, dentry->d_name.name, dentry->d_name.len);
> + prepend(&b, "/", 1);
> + return extract_string(&b);
> }
>
> /*
> * Write full pathname from the root of the filesystem into the buffer.
> */
> -static char *__dentry_path(const struct dentry *d, char *p, int buflen)
> +static char *__dentry_path(const struct dentry *d, struct prepend_buffer
> *p)
> {
> const struct dentry *dentry;
> - char *end;
> - int len, seq = 0;
> + struct prepend_buffer b;
> + int seq = 0;
>
> rcu_read_lock();
> restart:
> dentry = d;
> - end = p;
> - len = buflen;
> + b = *p;
> read_seqbegin_or_lock(&rename_lock, &seq);
> while (!IS_ROOT(dentry)) {
> const struct dentry *parent = dentry->d_parent;
>
> prefetch(parent);
> - if (!prepend_name(&end, &len, &dentry->d_name))
> + if (!prepend_name(&b, &dentry->d_name))
> break;
> -
> dentry = parent;
> }
> if (!(seq & 1))
> @@ -317,28 +325,29 @@ static char *__dentry_path(const struct dentry *d,
> char *p, int buflen)
> goto restart;
> }
> done_seqretry(&rename_lock, seq);
> - if (len == buflen)
> - prepend(&end, &len, "/", 1);
> - return len >= 0 ? end : ERR_PTR(-ENAMETOOLONG);
> + if (b.len == p->len)
> + prepend(&b, "/", 1);
> + return extract_string(&b);
> }
>
> char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
> {
> - char *p = buf + buflen;
> - prepend(&p, &buflen, "", 1);
> - return __dentry_path(dentry, p, buflen);
> + DECLARE_BUFFER(b, buf, buflen);
> +
> + prepend(&b, "", 1);
> + return __dentry_path(dentry, &b);
> }
> EXPORT_SYMBOL(dentry_path_raw);
>
> char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
> {
> - char *p = buf + buflen;
> + DECLARE_BUFFER(b, buf, buflen);
>
> if (unlikely(d_unlinked(dentry)))
> - prepend(&p, &buflen, "//deleted", 10);
> + prepend(&b, "//deleted", 10);
> else
> - prepend(&p, &buflen, "", 1);
> - return __dentry_path(dentry, p, buflen);
> + prepend(&b, "", 1);
> + return __dentry_path(dentry, &b);
> }
>
> static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path
> *root,
> @@ -386,24 +395,23 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned
> long, size)
> error = -ENOENT;
> if (!d_unlinked(pwd.dentry)) {
> unsigned long len;
> - char *cwd = page + PATH_MAX;
> - int buflen = PATH_MAX;
> + DECLARE_BUFFER(b, page, PATH_MAX);
>
> - prepend(&cwd, &buflen, "", 1);
> - if (prepend_path(&pwd, &root, &cwd, &buflen) > 0)
> - prepend(&cwd, &buflen, "(unreachable)", 13);
> + prepend(&b, "", 1);
> + if (prepend_path(&pwd, &root, &b) > 0)
> + prepend(&b, "(unreachable)", 13);
> rcu_read_unlock();
>
> - if (buflen < 0) {
> + if (b.len < 0) {
> error = -ENAMETOOLONG;
> goto out;
> }
>
> error = -ERANGE;
> - len = PATH_MAX + page - cwd;
> + len = PATH_MAX - b.len;
> if (len <= size) {
> error = len;
> - if (copy_to_user(buf, cwd, len))
> + if (copy_to_user(buf, b.buf, len))
> error = -EFAULT;
> }
> } else {
> --
> 2.11.0

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