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Re: [bug] :P modifier and symlink loops
Stephane Chazelas wrote on Sat, 01 Feb 2020 17:57 +0000:
> Ping:
Thanks for the ping. I've added this to Etc/BUGS so we don't forget
it. I worked on :P before, so I've added this to my list to
investigate further, but I don't know when I'll get to it.
> 2020-01-11 17:00:47 +0000, Stephane Chazelas:
> Hi,
>
> I've got the feeling it's been discussed before, but could not
> find it in the archives.
>
> $ ln -s loop /tmp/
> $ f=/tmp/loop strace ~/install/cvs/zsh/Src/zsh -c '$f:P'
> [...]
> readlink("/tmp/loop", "loop", 4096) = 4
> readlink("/tmp/loop", "loop", 4096) = 4
> [...]
> readlink("/tmp/loop", "loop", 4096) = 4
> readlink("/tmp/loop", "loop", 4096) = 4
> --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR,
> si_addr=0x7ffec7a345e0} ---
> +++ killed by SIGSEGV +++
>
> possibly stack overflow caused by unbound recursion or buffer
> overflow on /tmp/loop/loop... but the bigger question is what to
> do here.
>
> The ELOOP problem is usually addressed by giving up after an
> arbitrary number of symlinks has been resolved (regardless of
> whether there is indeed a loop or not) in the lookup of the
> file, but here $f:P *has* to expand to something, so what should
> that be?
>
> For instance, for
>
> cd /
> file=bin/../tmp/loop/../foo/.. above?
>
> The only thing I can think of is expand to:
>
> /tmp/loop/../foo/..
>
> (maybe done by first doing a stat(the-file); if it returns
> ELOOP, do a stat() at each stage of the resolution and give up
> on the first ELOOP).
>
> Any other idea?
The postcondition of :P is "no dot or dot-dot components and no symlinks".
When the loop is on the last path component (as in ${${:-/tmp/loop}:P}
and ${${:-/tmp/trap}:P} after «ln -s loop /tmp/trap») we could still print
a path to the loop symlink that meets the postcondition, except for the loop
symlink in the last path component.
However, in ${${:-"/tmp/loop/../foo"}} we can't meet the postcondition.
I think our options are either to throw an exception, like a glob with
no matches does, or to keep the additional components verbatim, as you
suggest.
Intuitively I lean towards the first option. We aren't a CGI script,
where PATH_INFO is to be expected. If we can't return a path without
dot and dot-dot components and without symlinks, we should raise an
error rather than continue silently. However, I'm open to alternatives.
I think the first option could be implemented along the lines of:
1. Call realpath($arg).
2. If it returns ELOOP, call realpath(${arg:h}) and append "/${arg:t}".
3. Otherwise, throw an exception (i.e., set errflag).
Cheers,
Daniel
P.S. Here's a quick test for the "loop in the last path component" case:
diff --git a/Test/D02glob.ztst b/Test/D02glob.ztst
index 3d7df94c9..a5657be59 100644
--- a/Test/D02glob.ztst
+++ b/Test/D02glob.ztst
@@ -742,6 +742,16 @@
>glob.tmp/secret-s111/ glob.tmp/secret-s111
>glob.tmp/secret-s444/ glob.tmp/secret-s444
+ ln -s loop glob.tmp/loop
+ ln -s loop glob.tmp/trap
+ {
+ $ZTST_testdir/../Src/zsh -fc 'echo $1:P' . glob.tmp/trap
+ } always {
+ rm -f glob.tmp/trap glob.tmp/loop
+ }
+-f:the ':P' modifier handles symlink loops in the last path component
+*>*/(trap|loop)
+
%clean
# Fix unreadable-directory permissions so ztst can clean up properly
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