RE: [PATCH RFC 1/3] fs: introduce helper d_path_fast()

[Justin He]

Hi Linus
I don't know how to display the attachment in this mail.
So I copied here below, together with one comment:
> -----Original Message-----
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Sent: Sunday, May 9, 2021 4:40 AM
> To: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Justin He <Justin.He@xxxxxxx>; Petr Mladek <pmladek@xxxxxxxx>; Steven
> Rostedt <rostedt@xxxxxxxxxxx>; Sergey Senozhatsky
> <senozhatsky@xxxxxxxxxxxx>; Andy Shevchenko
> <andriy.shevchenko@xxxxxxxxxxxxxxx>; Rasmus Villemoes
> <linux@xxxxxxxxxxxxxxxxxx>; Jonathan Corbet <corbet@xxxxxxx>; Al Viro
> <viro@xxxxxxxxxxxxxxxx>; Heiko Carstens <hca@xxxxxxxxxxxxx>; Vasily Gorbik
> <gor@xxxxxxxxxxxxx>; Christian Borntraeger <borntraeger@xxxxxxxxxx>; Eric
> W . Biederman <ebiederm@xxxxxxxxxxxx>; Darrick J. Wong
> <darrick.wong@xxxxxxxxxx>; Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>;
> Ira Weiny <ira.weiny@xxxxxxxxx>; Eric Biggers <ebiggers@xxxxxxxxxx>; Ahmed
> S. Darwish <a.darwish@xxxxxxxxxxxxx>; open list:DOCUMENTATION <linux-
> doc@xxxxxxxxxxxxxxx>; Linux Kernel Mailing List <linux-
> kernel@xxxxxxxxxxxxxxx>; linux-s390 <linux-s390@xxxxxxxxxxxxxxx>; linux-
> fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>
> Subject: Re: [PATCH RFC 1/3] fs: introduce helper d_path_fast()
>
> On Sat, May 8, 2021 at 12:13 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > Potentiallty delicate question is how to pass bptr/blen to the caller
> > of that helper...
>
> So I was thinking something like this patch..
>
> THIS IS TOTALLY UNTESTED!
>
> I've basically introduced a "struct prepend_buffer" that acts as that
> "end,len" pair and that gets passed around. It builds cleanly for me,
> which actually makes me fairly happy that it might even be close
> right, because the calling conventions would catch any mistake.
>
> It looks superficially sane to me, but again - untested.
>
> The patch ended up being bigger than I expected, but it's all fairly
> straightforward - just munging the calling conventions.
>
> Oh, and I did extract out the core of "prepend_path()" into
> "prepend_entries()" just to show how it would work.
>
>               Linus
> fs/d_path.c | 227 ++++++++++++++++++++++++++++++++----------------------------
> 1 file changed, 123 insertions(+), 104 deletions(-)
>
>diff --git a/fs/d_path.c b/fs/d_path.c
>index 270d62133996..47eb29524271 100644
>--- a/fs/d_path.c
>+++ b/fs/d_path.c
>@@ -8,13 +8,18 @@
> #include <linux/prefetch.h>
> #include "mount.h"
>
>-static int prepend(char **buffer, int *buflen, const char *str, int namelen)
>+struct prepend_buffer {
>+      char *ptr;
>+      int len;
>+};
>+
>+static int prepend(struct prepend_buffer *b, const char *str, int namelen)
> {
>-      *buflen -= namelen;
>-      if (*buflen < 0)
>+      b->len -= namelen;
>+      if (b->len < 0)
>               return -ENAMETOOLONG;
>-      *buffer -= namelen;
>-      memcpy(*buffer, str, namelen);
>+      b->ptr -= namelen;
>+      memcpy(b->ptr, str, namelen);
>       return 0;
> }
>
>@@ -35,16 +40,16 @@ static int prepend(char **buffer, int *buflen, const char *str, int namelen)
>  *
>  * Load acquire is needed to make sure that we see that terminating NUL.
>  */
>-static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
>+static int prepend_name(struct prepend_buffer *b, const struct qstr *name)
> {
>       const char *dname = smp_load_acquire(&name->name); /* ^^^ */
>       u32 dlen = READ_ONCE(name->len);
>       char *p;
>
>-      *buflen -= dlen + 1;
>-      if (*buflen < 0)
>+      b->len -= dlen + 1;
>+      if (b->len < 0)
>               return -ENAMETOOLONG;
>-      p = *buffer -= dlen + 1;
>+      p = b->ptr -= dlen + 1;
>       *p++ = '/';
>       while (dlen--) {
>               char c = *dname++;
>@@ -55,6 +60,50 @@ static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
>       return 0;
> }
>
>+static inline int prepend_entries(struct prepend_buffer *b, const struct path *path, const struct path *root, struct mount *mnt)
>+{
>+      struct dentry *dentry = path->dentry;
>+      struct vfsmount *vfsmnt = path->mnt;
>+
>+      while (dentry != root->dentry || vfsmnt != root->mnt) {
>+              int error;
>+              struct dentry * parent;
>+
>+              if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
>+                      struct mount *parent = READ_ONCE(mnt->mnt_parent);
>+                      struct mnt_namespace *mnt_ns;
>+
>+                      /* Escaped? */
>+                      if (dentry != vfsmnt->mnt_root)
>+                              return 3;
>+
>+                      /* Global root? */
>+                      if (mnt != parent) {
>+                              dentry = READ_ONCE(mnt->mnt_mountpoint);
>+                              mnt = parent;
>+                              vfsmnt = &mnt->mnt;
>+                              continue;
>+                      }
>+                      mnt_ns = READ_ONCE(mnt->mnt_ns);
>+                      /* open-coded is_mounted() to use local mnt_ns */
>+                      if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
>+                              return 1;       // absolute root
>+
>+                      return 2;               // detached or not attached yet
>+                      break;
>+              }
>+              parent = dentry->d_parent;
>+              prefetch(parent);
>+              error = prepend_name(b, &dentry->d_name);
>+              if (error)
>+                      break;
>+
>+              dentry = parent;
>+      }
>+      return 0;
>+}
>+
>+
> /**
>  * prepend_path - Prepend path string to a buffer
>  * @path: the dentry/vfsmount to report
>@@ -74,15 +123,12 @@ static int 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 *orig)
> {
>-      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:
>@@ -90,50 +136,12 @@ static int prepend_path(const struct path *path,
>       seq = 0;
>       rcu_read_lock();
> restart:
>-      bptr = *buffer;
>-      blen = *buflen;
>-      error = 0;
>-      dentry = path->dentry;
>-      vfsmnt = path->mnt;
>-      mnt = real_mount(vfsmnt);
>+      b = *orig;
>+      mnt = real_mount(path->mnt);
>       read_seqbegin_or_lock(&rename_lock, &seq);
>-      while (dentry != root->dentry || vfsmnt != root->mnt) {
>-              struct dentry * parent;
>
>-              if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
>-                      struct mount *parent = READ_ONCE(mnt->mnt_parent);
>-                      struct mnt_namespace *mnt_ns;
>+      error = prepend_entries(&b, path, root, mnt);
>
>-                      /* Escaped? */
>-                      if (dentry != vfsmnt->mnt_root) {
>-                              bptr = *buffer;
>-                              blen = *buflen;
>-                              error = 3;
>-                              break;
>-                      }
>-                      /* Global root? */
>-                      if (mnt != parent) {
>-                              dentry = READ_ONCE(mnt->mnt_mountpoint);
>-                              mnt = parent;
>-                              vfsmnt = &mnt->mnt;
>-                              continue;
>-                      }
>-                      mnt_ns = READ_ONCE(mnt->mnt_ns);
>-                      /* open-coded is_mounted() to use local mnt_ns */
>-                      if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
>-                              error = 1;      // absolute root
>-                      else
>-                              error = 2;      // detached or not attached yet
>-                      break;
>-              }
>-              parent = dentry->d_parent;
>-              prefetch(parent);
>-              error = prepend_name(&bptr, &blen, &dentry->d_name);
>-              if (error)
>-                      break;
>-
>-              dentry = parent;
>-      }
>       if (!(seq & 1))
>               rcu_read_unlock();
>       if (need_seqretry(&rename_lock, seq)) {
>@@ -150,14 +158,17 @@ static int prepend_path(const struct path *path,
>       }
>       done_seqretry(&mount_lock, m_seq);
>
>-      if (error >= 0 && bptr == *buffer) {
>-              if (--blen < 0)
>+      // Escaped? No path
>+      if (error == 3)
>+              b = *orig;
>+
>+      if (error >= 0 && b.ptr == orig->ptr) {
>+              if (--b.len < 0)
>                       error = -ENAMETOOLONG;
>               else
>-                      *--bptr = '/';
>+                      *--b.ptr = '/';
>       }
>-      *buffer = bptr;
>-      *buflen = blen;
>+      *orig = b;
>       return error;
> }
>
>@@ -181,34 +192,34 @@ char *__d_path(const struct path *path,
>              const struct path *root,
>              char *buf, int buflen)
> {
>-      char *res = buf + buflen;
>+      struct prepend_buffer b = { buf + buflen, buflen };
>       int error;
>
>-      prepend(&res, &buflen, "\0", 1);
>-      error = prepend_path(path, root, &res, &buflen);
>+      prepend(&b, "\0", 1);
>+      error = prepend_path(path, root, &b);
>
>       if (error < 0)
>               return ERR_PTR(error);
>       if (error > 0)
>               return NULL;
>-      return res;
>+      return b.ptr;
> }
>
> char *d_absolute_path(const struct path *path,
>              char *buf, int buflen)
> {
>       struct path root = {};
>-      char *res = buf + buflen;
>+      struct prepend_buffer b = { buf + buflen, buflen };
>       int error;
>
>-      prepend(&res, &buflen, "\0", 1);
>-      error = prepend_path(path, &root, &res, &buflen);
>+      prepend(&b, "\0", 1);
>+      error = prepend_path(path, &root, &b);
>
>       if (error > 1)
>               error = -EINVAL;
>       if (error < 0)
>               return ERR_PTR(error);
>-      return res;
>+      return b.ptr;
> }
>
> /*
>@@ -216,21 +227,21 @@ char *d_absolute_path(const struct path *path,
>  */
> static int path_with_deleted(const struct path *path,
>                            const struct path *root,
>-                           char **buf, int *buflen)
>+                           struct prepend_buffer *b)
> {
>-      prepend(buf, buflen, "\0", 1);
>+      prepend(b, "\0", 1);
>       if (d_unlinked(path->dentry)) {
>-              int error = prepend(buf, buflen, " (deleted)", 10);
>+              int error = prepend(b, " (deleted)", 10);
>               if (error)
>                       return error;
>       }
>
>-      return prepend_path(path, root, buf, buflen);
>+      return prepend_path(path, root, b);
> }
>
>-static int prepend_unreachable(char **buffer, int *buflen)
>+static int prepend_unreachable(struct prepend_buffer *b)
> {
>-      return prepend(buffer, buflen, "(unreachable)", 13);
>+      return prepend(b, "(unreachable)", 13);
> }
>
> static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
>@@ -261,7 +272,7 @@ 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;
>+      struct prepend_buffer b = { buf + buflen, buflen };
>       struct path root;
>       int error;
>
>@@ -282,12 +293,12 @@ char *d_path(const struct path *path, char *buf, int buflen)
>
>       rcu_read_lock();
>       get_fs_root_rcu(current->fs, &root);
>-      error = path_with_deleted(path, &root, &res, &buflen);
>+      error = path_with_deleted(path, &root, &b);
>       rcu_read_unlock();
>
>       if (error < 0)
>-              res = ERR_PTR(error);
>-      return res;
>+              return ERR_PTR(error);
>+      return b.ptr;
> }
> EXPORT_SYMBOL(d_path);
>
>@@ -314,13 +325,14 @@ char *dynamic_dname(struct dentry *dentry, char *buffer, int buflen,
>
> char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
> {
>-      char *end = buffer + buflen;
>+      struct prepend_buffer b = { buffer + buflen, buflen };
>+
>       /* these dentries are never renamed, so d_lock is not needed */
>-      if (prepend(&end, &buflen, " (deleted)", 11) ||
>-          prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) ||
>-          prepend(&end, &buflen, "/", 1))
>-              end = ERR_PTR(-ENAMETOOLONG);
>-      return end;
>+      if (prepend(&b, " (deleted)", 11) ||
>+          prepend(&b, dentry->d_name.name, dentry->d_name.len) ||
>+          prepend(&b, "/", 1))
>+              return ERR_PTR(-ENAMETOOLONG);
>+      return b.ptr;
> }
>
> /*
>@@ -329,8 +341,9 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
> static char *__dentry_path(const struct dentry *d, char *buf, int buflen)
> {
>       const struct dentry *dentry;
>-      char *end, *retval;
>-      int len, seq = 0;
>+      struct prepend_buffer b;
>+      char *retval;
>+      int seq = 0;
>       int error = 0;
>
>       if (buflen < 2)
>@@ -339,22 +352,22 @@ static char *__dentry_path(const struct dentry *d, char *buf, int buflen)
>       rcu_read_lock();
> restart:
>       dentry = d;
>-      end = buf + buflen;
>-      len = buflen;
>-      prepend(&end, &len, "\0", 1);
>+      b.ptr = buf + buflen;
>+      b.len = buflen;
>+      prepend(&b, "\0", 1);
>       /* Get '/' right */
>-      retval = end-1;
>+      retval = b.ptr-1;
>       *retval = '/';
>       read_seqbegin_or_lock(&rename_lock, &seq);
>       while (!IS_ROOT(dentry)) {
>               const struct dentry *parent = dentry->d_parent;
>
>               prefetch(parent);
>-              error = prepend_name(&end, &len, &dentry->d_name);
>+              error = prepend_name(&b, &dentry->d_name);
>               if (error)
>                       break;
>
>-              retval = end;
>+              retval = b.ptr;
>               dentry = parent;
>       }
>       if (!(seq & 1))
>@@ -379,16 +392,23 @@ EXPORT_SYMBOL(dentry_path_raw);
>
> char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
> {
>-      char *p = NULL;
>+      struct prepend_buffer b = { buf + buflen, buflen };
>       char *retval;
>+      char *p = NULL;
>
>       if (d_unlinked(dentry)) {
>-              p = buf + buflen;
>-              if (prepend(&p, &buflen, "//deleted", 10) != 0)
>+              if (prepend(&b, "//deleted", 10) != 0)
>                       goto Elong;
>-              buflen++;
>+
>+              // save away beginning of "//deleted" string
>+              // and let "__dentry_path()" overwrite one byte
>+              // with the terminating NUL that we'll restore
>+              // below.
>+              p = b.ptr;
>+              b.ptr++;
>+              b.len++;
>       }
>-      retval = __dentry_path(dentry, buf, buflen);
>+      retval = __dentry_path(dentry, b.ptr, b.len);

I didn't quite understand the logic here. Seems it is not equal to
the previous. Should it be s/b.ptr/buf here? Otherwise, in __dentry_path,
it will use the range [b.ptr, b.ptr+b.len] instead of [buf, buf+b.len].
Am I missing anything here?

--
Cheers,
Justin (Jia He)

>       if (!IS_ERR(retval) && p)
>               *p = '/';       /* restore '/' overriden with '\0' */
>       return retval;
>@@ -441,11 +461,10 @@ 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;
>+              struct prepend_buffer b = { page + PATH_MAX, PATH_MAX };
>
>-              prepend(&cwd, &buflen, "\0", 1);
>-              error = prepend_path(&pwd, &root, &cwd, &buflen);
>+              prepend(&b, "\0", 1);
>+              error = prepend_path(&pwd, &root, &b);
>               rcu_read_unlock();
>
>               if (error < 0)
>@@ -453,16 +472,16 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
>
>               /* Unreachable from current root */
>               if (error > 0) {
>-                      error = prepend_unreachable(&cwd, &buflen);
>+                      error = prepend_unreachable(&b);
>                       if (error)
>                               goto out;
>               }
>
>               error = -ERANGE;
>-              len = PATH_MAX + page - cwd;
>+              len = PATH_MAX + page - b.ptr;
>               if (len <= size) {
>                       error = len;
>-                      if (copy_to_user(buf, cwd, len))
>+                      if (copy_to_user(buf, b.ptr, len))
>                               error = -EFAULT;
>               }
>       } else {
>
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