[PATCH v2 0/3] Support opening a symlink with O_PATH | O_NOFOLLOW
Mon Jan 13 17:44:00 GMT 2020
On 1/13/2020 12:24 PM, Eric Blake wrote:
> On 1/13/20 10:53 AM, Ken Brown wrote:
>> On 1/13/2020 10:28 AM, Corinna Vinschen wrote:
>>> Hi Ken,
>>> On Dec 29 17:56, Ken Brown wrote:
>>>> Currently, opening a symlink with O_NOFOLLOW fails with ELOOP.
>>>> Following Linux, the first patch in this series allows the call to
>>>> succeed if O_PATH is also specified.
>>> O_PATH (since Linux 2.6.39)
>>> Obtain a file descriptor that can be used for two purposes: to
>>> indicate a location in the filesystem tree and to perform opera‐
>>> tions that act purely at the file descriptor level. The file
>>> itself is not opened, and other file operations (e.g., read(2),
>>> write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2), mmap(2))
>>> fail with the error EBADF.
>>> ^^^^^^^^^ ^^^^^
> On BSD systems, you are able to run lchmod to change permissions on a symlink
> (with effect on who is able to follow that symlink during pathname resolution);
> Linux does not support that, and POSIX does not mandate support for that, so
> fchmodat() is allowed to fail on symlinks even while fchownat() is required to
> work on symlinks.
>>> That'd from the current F31 man pages.
>>>> Am I missing something?
>>> Good question. Let me ask in return, did *I* now miss something?
>> I don't think so. I think we agree, although maybe I didn't express myself
>> clearly enough for that to be obvious. What confused me was the following
>> paragraph further down in the open(2) man page (still discussing O_PATH):
>> If pathname is a symbolic link and the O_NOFOLLOW flag is also
>> specified, then the call returns a file descriptor referring
>> to the symbolic link. This file descriptor can be used as the
>> dirfd argument in calls to fchownat(2), fstatat(2), linkat(2),
>> and readlinkat(2) with an empty pathname to have the calls
>> operate on the symbolic link.
>> I don't know why they include fchownat here, since the resulting call would fail
>> with EBADF. So I didn't implement that in my patch series.
> I'm not sure if the question here is about fchownat() (where you CAN change
> owner of a symlink on Linux, same as with lchown())
Yes, the question is about fchownat. Are you saying you can change the owner
even if the symlink was opened with O_PATH?
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