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Date: Tue, 24 Nov 2020 01:30:49 -0500
From: Rich Felker <dalias@...c.org>
To: Alexey Izbyshev <izbyshev@...ras.ru>
Cc: musl@...ts.openwall.com
Subject: Re: realpath without procfs -- should be ready for inclusion
On Tue, Nov 24, 2020 at 08:13:56AM +0300, Alexey Izbyshev wrote:
> On 2020-11-24 07:26, Rich Felker wrote:
> >On Tue, Nov 24, 2020 at 06:39:59AM +0300, Alexey Izbyshev wrote:
> >>On 2020-11-23 23:53, Rich Felker wrote:
> >>>On Mon, Nov 23, 2020 at 01:56:33PM -0500, Rich Felker wrote:
> >>>>On Sun, Nov 22, 2020 at 10:19:33PM -0500, Rich Felker wrote:
> >>>>--- realpath8.c 2020-11-22 17:52:17.586481571 -0500
> >>>>+++ realpath9.c 2020-11-23 13:55:06.808458893 -0500
> >>>>@@ -19,7 +19,7 @@
> >>>> char *output = resolved ? resolved : buf;
> >>>> size_t p, q, l, cnt=0;
> >>>>
> >>>>- l = strnlen(filename, sizeof stack + 1);
> >>>>+ l = strnlen(filename, sizeof stack);
> >>>> if (!l) {
> >>>> errno = ENOENT;
> >>>> return 0;
> >>>>@@ -80,11 +80,16 @@
> >>>> return 0;
> >>>> }
> >>>> if (k==p) goto toolong;
> >>>>+ if (!k) {
> >>>>+ errno = ENOENT;
> >>>>+ return 0;
> >>>>+ }
> >>>> if (++cnt == SYMLOOP_MAX) {
> >>>> errno = ELOOP;
> >>>> return 0;
> >>>> }
> >>>> p -= k;
> >>>>+ if (stack[k-1]=='/') p++;
> >>>> memmove(stack+p, stack, k);
> >>>
> >>>This is wrong and needs further consideration.
> >>>
> >>Yes, now memmove() overwrites NUL if p was at the end and stack[k-1]
> >>== '/'. Is it true per POSIX that "rr/home" must resolve to "//home"
> >>if "rr" -> "//"?
> >
> >I don't think // is even required be distinct from /, just permitted,
> >but I think allowing it in userspace and handling it consistently is
> >the right behavior in case you ever run on a kernel that does make use
> >of the distinction.
> >
> >>If so, maybe something like the following instead:
> >>
> >>+ while (stack[p] == '/') p++;
> >>+ if (stack[p] && stack[k-1] != '/') p--;
> >> p -= k;
> >>- if (stack[k-1]=='/') p++;
> >
> >Rather just:
> >
> > /* If link contents end in /, strip any slashes already on
> > * stack to avoid /->// or //->/// or spurious toolong. */
> > if (stack[k-1]=='/') while (stack[p]=='/') p++;
> >
> >should work (before the p-=k;)
> >
> Yes, that looks good.
>
> >>I've also noticed other issues to be fixed, per POSIX:
> >>
> >>* ENOENT should be returned if filename is NULL
> >
> >Rather it looks like it's:
> >
> > [EINVAL] The file_name argument is a null pointer.
> >
> >ENOENT is only for empty string or ENOENT somewhere in the path
> >traversal process.
> >
> Uh, yes, that was bad copy-paste or something.
>
> >>* ENOTDIR should be returned if the last component is not a
> >>directory and the path has one or more trailing slashes
> >
> >Yes, that's precisely what I've been working on the past couple hours.
> >I think you missed but .. will also erase a path component that's not
> >a dir (e.g. /dev/null/.. -> /dev) and these are both instances of a
> >common problem. I thought use of readlink covered all the ENOTDIR
> >cases but it doesn't when the next component isn't covered by readlink
> >or isn't present at all.
> >
> Yes, initially I forgot about this whole ENOTDIR issue completely,
> and after noticing the problem with the last component, didn't look
> further.
I think before this goes upstream we should have a good set of
testcases that can be contributed to libc-test. Do you have ideas for
coverage? Some that come to mind:
- Absolute argument starting with /, //, and ///
- Absolute symlink target starting with /, //, and ///
- Final / after symlink-to-dir, dir, symlink-to-nondir, nondir
- Final / in link contents after [the above]
- Initial .. in argument, cwd root or non-root
- Initial .. in symlink target, symlink in root or non-root
- Initial ...
- .. following symlink-to-dir, dir, symlink-to-nondir, nondir
- More .. than path depth
- Null argument
- Empty string argument
- Empty string link contents (testable only with seccomp hack)
- Argument valid abs path exact length PATH_MAX-1
- Argument valid rel path exact length PATH_MAX-1 to short abs path
Some of these require namespace gymnastics to set up without running
the tests as root.
Rich
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