On 2025-08-21, Askar Safin [off-list ref] wrote:

There is a convention: you can pass invalid fd (such as -1) as dfd to *at-syscalls to enforce that the path is absolute.
This is documented. "man openat" says: "Specifying an invalid file descriptor number in dirfd can be used as a means to ensure that pathname is absolute".
But fsconfig with FSCONFIG_SET_PATH breaks this convention due to this line: https://elixir.bootlin.com/linux/v6.16/source/fs/fsopen.c#L377 .
I think this is a bug, and it should be fixed in kernel. Also, it is possible there are a lot of similarly buggy syscalls. All of them should be fixed,
and moreover a warning should be added to https://docs.kernel.org/process/adding-syscalls.html . And then new fsconfig behavior should be documented.
(Of course, I'm not saying that *you* should do all these. I'm just saying that this bug exists.) (I tested this.)

Indeed, good catch! I think we discussed this before --
FSCONFIG_SET_PATH actually doesn't work with any parameters today so
it's not very surprising nobody has noticed this until now. I'll include
it in the set of fixes I have for fscontext.

(FWIW, the convention I see more commonly is -EBADF but that's just a
stylistic I suppose.)

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/