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Date: Tue, 24 Nov 2020 08:13:56 +0300
From: Alexey Izbyshev <izbyshev@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: realpath without procfs -- should be ready for inclusion
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.
Alexey
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