linux-kernel - Re: [PATCHv4 3/8] mm: Pass down mm_struct to untagged

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Message-ID: <CAG_fn=WgyitSd9h2ni2xpBBvgnoGTcwZOpWyNE5QRSRn+PcC=A@mail.gmail.com>
Date:   Tue, 5 Jul 2022 17:42:21 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv4 3/8] mm: Pass down mm_struct to untagged_addr()

Kirill,


> diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> index feeb935a2299..abc096a68f05 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -97,7 +97,7 @@ long strnlen_user(const char __user *str, long count)
>                 return 0;
>
>         max_addr = TASK_SIZE_MAX;
> -       src_addr = (unsigned long)untagged_addr(str);
> +       src_addr = (unsigned long)untagged_addr(current->mm, str);

In a downstream kernel with LAM disabled I'm seeing current->mm being
NULL at this point, because strnlen_user() is being called by
kdevtmpfs.
IIUC current->mm is only guaranteed to be non-NULL in the userspace
process context, whereas untagged_addr() may get called in random
places.

Am I missing something?



-- 
Alexander Potapenko
Software Engineer

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