linux-kernel - Re: [PATCH] kernel/groups.c: use bsearch library function

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Message-ID: <875zkxydfy.fsf@m3y3r.de>
Date:   Wed, 09 Oct 2019 20:35:29 +0200
From:   Thomas Meyer <thomas@...3r.de>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/groups.c: use bsearch library function

Rasmus Villemoes <linux@...musvillemoes.dk> writes:

> On 07/10/2019 21.26, Thomas Meyer wrote:
>> commit b7b2562f7252 ("kernel/groups.c: use sort library function")
>> introduced the sort library function.
>> also use the bsearch library function instead of open-coding the binary
>> search.

Hi,

> Yes, but please note the difference between sorting the group_info and
> searching it: The former is done quite rarely - the setgroups syscall is
> used roughly once per login-session.
>
> But the searching of that structure is done more or less every time a
> user accesses a file not owned by that user (e.g., any time a normal
> user accesses anything in /usr) - at least if I'm reading
> acl_permission_check() right.
>
> So using a callback-based interface, especially in a post-spectre world,
> may have a somewhat large performance impact.

okay, so the code is duplicated for performance reasons? nothing a
compiler can inline, I guess.

so what about a comment instead:

diff --git a/kernel/groups.c b/kernel/groups.c
index daae2f2dc6d4f..46b5d4cd53c2e 100644
--- a/kernel/groups.c
+++ b/kernel/groups.c
@@ -93,7 +93,7 @@ void groups_sort(struct group_info *group_info)
 }
 EXPORT_SYMBOL(groups_sort);
 
-/* a simple bsearch */
+/* duplicate code from lib/bsearch.c for performance reasons */
 int groups_search(const struct group_info *group_info, kgid_t grp)
 {
        unsigned int left, right;


Mfg
thomas