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Re: [patch] libstdc++/67173 Fix filesystem::canonical for Solaris 10.

On 17/09/15 09:37 -0600, Martin Sebor wrote:
On 09/17/2015 05:16 AM, Jonathan Wakely wrote:
On 16/09/15 17:42 -0600, Martin Sebor wrote:
I see now the first exists test will detect symlink loops in
the original path. But I'm not convinced there isn't a corner
case that's subject to a TOCTOU race condition between the first
exists test and the while loop during which a symlink loop can
be introduced.

Suppose we call the function with /foo/bar as an argument and
the path exists and contains no symlinks. result is / and cmpts
is set to { foo, bar }. Just as the loop is entered, /foo/bar
is replaced with a symlink containing /foo/bar. The loop then
proceeds like so:

1. The first iteration removes foo from cmpts and sets result
to /foo. cmpts is { bar }.

2. The second iteration removes bar from cmpts, sets result to
/foo/bar, determines it's a symlink, reads its contents, sees
it's an absolute pathname and replaces result with /. It then
inserts the symlink's components { foo, bar } into cmpts. cmpts
becomes { foo, bar }. exists(result) succeeds.

3. The next iteration of the loop has the same initial state
as the first.

But I could have very easily missed something that takes care
of this corner case. If I did, sorry for the false alarm!

No, you're right. The TS says such filesystem races are undefined:

but it would be nice to fail gracefully rather than DOS the

The simplest approach would be to increment a counter every time we
follow a symlink, and if it reaches some limit decide something is
wrong and fail with ELOOP.

I don't see how anything else can be 100% bulletproof, because a truly
evil attacker could just keep altering the contents of symlinks so we
keep ping-ponging between two or more paths. If we keep track of paths
we've seen before the attacker could just keep changing the contents
to a unique path each time, that initially exists as a file, but by
the time we get to is_symlink() its become a symlink to a new path.

So if we use a counter, what's a sane maximum? Is MAXSYMLINKS in
<sys/param.h> the value the kernel uses? 20 seems quite low, I was
thinking of a much higher number.

Yes, it is a corner case, and it's not really avoidable in the case
of hard links. For symlinks, POSIX defines the SYMLOOP_MAX constant
as the maximum, with the _SC_SYMLOOP_MAX and _PC_SYMLOOP_MAX
sysconf and pathconf variables. Otherwise 40 seems reasonable.

With this, I'll let you get back to work -- I think we've beat this
function to death ;)

Here's what I committed. Similar to the last patch, but using the new
is_dot and is_dotdot helpers.

Attachment: patch-fs-3.txt
Description: Text document

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