bug#51327: 28.0.60; emacsclient warns about XDG_RUNTIME_DIR when starting daemon on-demand

[Jim Porter]

On 10/21/2021 9:58 PM, Jim Porter wrote:
> Normally, when running `emacsclient --alternate-editor=""' with no Emacs 
> server running, it will run `emacs --daemon' and then connect to it. In 
> Emacs 28, it will also issue the following warning:
>
>    Should XDG_RUNTIME_DIR='/run/user/1000' be in the environment?
>    (Be careful: XDG_RUNTIME_DIR is security-related.)
>
> However, XDG_RUNTIME_DIR *is* set in my environment, so it shouldn't be 
> warning me about it.
>
> I believe this is due to the fix for bug#33847 (see commit 
> 007744dd0404d6febca88b00c22981cc630fb8c0). That bug asked for 
> emacsclient to look in both XDG_RUNTIME_DIR and TMPDIR to find the 
> server socket, in order to accommodate the case where `emacs --daemon' 
> is started when XDG_RUNTIME_DIR is unset, but *is* set when running 
> `emacsclient'.

Attached is a patch that should fix this by skipping the TMPDIR check 
whenever a) we have an alternate editor and b) XDG_RUNTIME_DIR is set. 
This has the benefit of supporting the use case in bug#33847 as well as 
users who start the Emacs daemon on-demand.

The only flaw I can think of with this method is that it would still be 
technically possible to perform a symlink attack against a user who runs 
`emacs --daemon' explicitly with XDG_RUNTIME_DIR set, and then runs 
`emacsclient' without an alternate editor set. However, this would 
require the attacker to be able to kill the `emacs --daemon' process 
somehow so that emacsclient falls back to looking in TMPDIR. I'm not 
sure that's a realistic attack vector, but I thought I'd mention it for 
completeness.

0001-Prevent-symlink-attacks-in-emacsclient-when-an-alter.patch
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