linux-kernel - Re: PROBLEM: fanotify

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Date:   Tue, 24 Nov 2020 11:20:33 +0100
From:   Jan Kara <jack@...e.cz>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Paweł Jasiak <pawel@...iak.xyz>,
        Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Brian Gerst <brgerst@...il.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: PROBLEM: fanotify_mark EFAULT on x86

On Tue 24-11-20 09:45:07, Borislav Petkov wrote:
> On Mon, Nov 23, 2020 at 11:46:51PM +0100, Paweł Jasiak wrote:
> > On 23/11/20, Jan Kara wrote:
> > > OK, with a help of Boris Petkov I think I have a fix that looks correct
> > > (attach). Can you please try whether it works for you? Thanks!
> > 
> > Unfortunately I am getting a linker error.
> > 
> > ld: arch/x86/entry/syscall_32.o:(.rodata+0x54c): undefined reference to `__ia32_sys_ia32_fanotify_mark'
> 
> Because CONFIG_COMPAT is not set in your .config.
> 
> Jan, look at 121b32a58a3a and especially this hunk
> 
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 9b294c13809a..b8f89f78b8cd 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -53,6 +53,8 @@ obj-y                 += setup.o x86_init.o i8259.o irqinit.o
>  obj-$(CONFIG_JUMP_LABEL)       += jump_label.o
>  obj-$(CONFIG_IRQ_WORK)  += irq_work.o
>  obj-y                  += probe_roms.o
> +obj-$(CONFIG_X86_32)   += sys_ia32.o
> +obj-$(CONFIG_IA32_EMULATION)   += sys_ia32.o
> 
> how it enables the ia32 syscalls depending on those config items. Now,
> you have
> 
>  #ifdef CONFIG_COMPAT
> -COMPAT_SYSCALL_DEFINE6(fanotify_mark,
> +SYSCALL_DEFINE6(ia32_fanotify_mark,
> 
> which is under CONFIG_COMPAT which is not set in Paweł's config.
> 
> config COMPAT
>         def_bool y
>         depends on IA32_EMULATION || X86_X32
> 
> but it depends on those two config items.
> 
> However, it looks to me like ia32_fanotify_mark's definition should be
> simply under CONFIG_X86_32 because:
> 
> IA32_EMULATION is 32-bit emulation on 64-bit kernels
> X86_X32 is for the x32 ABI

Thanks for checking! I didn't realize I needed to change the ifdefs as well
(I missed that bit in 121b32a58a3a). So do I understand correctly that
whenever the kernel is 64-bit, 64-bit syscall args (e.g. defined as u64) are
passed just fine regardless of whether the userspace is 32-bit or not?

Also how about other 32-bit archs? Because I now realized that
CONFIG_COMPAT as well as the COMPAT_SYSCALL_DEFINE6() is also utilized by
other 32-bit archs (I can see a reference to compat_sys_fanotify_mark e.g.
in sparc, powerpc, and other args). So I probably need to actually keep
that for other archs but do the modification only for x86, don't I?

So something like attached patch?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-fanotify-Fix-fanotify_mark-on-32-bit-x86.patch" of type "text/x-patch" (2494 bytes)