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Re: `pwd -P` with systemd-homed causes inconsistent cwd state

X-seq: zsh-workers 52238
From: Kyle Laker <kyle@laker.email>
To: Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>
Cc: zsh-workers@xxxxxxx
Subject: Re: `pwd -P` with systemd-homed causes inconsistent cwd state
Date: Sat, 21 Oct 2023 12:26:37 -0400
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References: <f31c3d78-11de-4aac-81d7-c061916a9108@laker.email> <CAH+w=7YepkrZ1EYkovCvH-Qib4LCqFMGaw+j3EG8Cn3bTaE+XQ@mail.gmail.com>

On 10/21/23 00:05, Bart Schaefer wrote:

On Fri, Oct 20, 2023 at 8:18 PM Kyle Laker <kyle@laker.email> wrote:

When you run `pwd -P`, zsh invokes the getcwd() system call and prints
whatever it returns.  That's literally all it does.  So, although I
don't have BTRFS to test with, this looks from here like a problem
with getcwd() internally changing the process current directory to the
root when crossing a BTRFS link.  That would be invisible to $PWD and
therefore to the pwd command (without -P).

Thanks for the context! So it seems like the issue is in thenon-getcwd() zgetdir() code path. Checking against master, this issue isno longer present with a default configure after workers/50287 enabledusing getcwd() by default just after the 5.9 release. getcwd() seems tohandle this case correctly.

For situations where getcwd() isn't used, the actual errant `cd`inghappens at the end of the `while` loop in zgetdir(). This continuouslycalls `chdir("..")`. Eventually, this puts us at `/`. Typically, thecall to `zchdir(buf + pos)` would take us back to the directory westarted in; however, because ZSH determined the path was`/home/kyle.homedir` (using the homed image path) instead of`/home/kyle` (the actual mounted home directory), that fails. The`.homedir` folder is owned by nobody:nobody so when zchdir() callschdir(3) the latter fails with an EACCES error. This leaves the processin `/`, where it was put by zgetdir() but without any of the otherenvironment variables updated.

I'm not sure how these btrfs homedir mounts can be determined insituations like this (or how valuable it would be since nearly everyonewith btrfs probably has a working getcwd(3)). But perhaps if the finalzchdir() fails, zgetdir() could chdir(3) back to the original pathrather than the resolved path?

Follow-Ups:
- Re: `pwd -P` with systemd-homed causes inconsistent cwd state
  - From: Bart Schaefer

References:
- `pwd -P` with systemd-homed causes inconsistent cwd state
  - From: Kyle Laker
- Re: `pwd -P` with systemd-homed causes inconsistent cwd state
  - From: Bart Schaefer

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