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d_path: regularize handling of root dentry in __dentry_path()

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All path-forming primitives boil down to sequence of prepend_name()
on dentries encountered along the way toward root.  Each time we prepend
/ + dentry name to the buffer.  Normally that does exactly what we want,
but there's a corner case when we don't call prepend_name() at all (in case
of __dentry_path() that happens if we are given root dentry).  We obviously
want to end up with "/", rather than "", so this corner case needs to be
handled.

__dentry_path() used to manually put '/' in the end of buffer before
doing anything else, to be overwritten by the first call of prepend_name()
if one happens and to be left in place if we don't call prepend_name() at
all.  That required manually checking that we had space in the buffer
(prepend_name() and prepend() take care of such checks themselves) and lead
to clumsy keeping track of return value.

A better approach is to check if the main loop has added anything
into the buffer and prepend "/" if it hasn't.  A side benefit of using prepend()
is that it does the right thing if we'd already run out of buffer, making
the overflow-handling logics simpler.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2021-05-17 21:19:35 -04:00
parent 3a291c974c
commit 3acca04326
1 changed files with 5 additions and 16 deletions
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21
fs/d_path.c
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@ -329,31 +329,22 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
static char *__dentry_path(const struct dentry *d, char *p, int buflen)
{
const struct dentry *dentry;
char *end, *retval;
char *end;
int len, seq = 0;
int error = 0;
if (buflen < 1)
goto Elong;
rcu_read_lock();
restart:
dentry = d;
end = p;
len = buflen;
/* Get '/' right */
retval = end-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);
if (error)
if (unlikely(prepend_name(&end, &len, &dentry->d_name) < 0))
break;
retval = end;
dentry = parent;
}
if (!(seq & 1))
@ -363,11 +354,9 @@ restart:
goto restart;
}
done_seqretry(&rename_lock, seq);
if (error)
goto Elong;
return retval;
Elong:
return ERR_PTR(-ENAMETOOLONG);
if (len == buflen)
prepend(&end, &len, "/", 1);
return len >= 0 ? end : ERR_PTR(-ENAMETOOLONG);
}
char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)