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Date: Mon, 23 Nov 2020 23:26:46 -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 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;)
> >> }
> >>
> >>@@ -95,7 +100,8 @@
> >> l = strlen(stack);
> >> /* Cancel any initial .. components. */
> >> p = 0;
> >>- while (q-p>=2 && at_dotdot(output+p+2, p+2)) {
> >>+ while (output[p]=='.' && output[p+1]=='.'
> >>+ && (!output[p+2] || output[p+2]=='/')) {
> >> while(l>1 && stack[l-1]!='/') l--;
> >> if (l>1) l--;
> >> p += 2;
> >
> >OK, I have a better improvement for this: counting the number of
> >levels of .. as they're built at the head of output. Then it's just
> >while (nup--) here, and the condition for canceling .. in the first
> >loop no longer needs any string inspection; it's just (q>3*nup).
> >
> Sounds good.
>
> I've missed the last time that the immediately following code is
> also broken:
>
> > if (q-p && stack[l-1]!='/') output[--p] = '/';
>
> It will underflow the output in case of a simple relative path that
> doesn't start with "..".
Thanks. This logic just looks wrong; I'll rework it.
> 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.
> * 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.
It's trivial to fix with a check after each component but that doubles
the number of syscalls and mostly isn't necessary. I have a reworked
draft to fix the problem by advancing over /(/|./|.$)* rather than just
/+ after each component, so that we can lookahead and do an extra
readlink in the cases that need it.
Rich
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