linux-kernel - Re: [PATCH v5 03/10] kexec_file: factor out arch_kexec_kernel

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Date:   Fri, 13 Oct 2017 17:47:23 +0900
From:   AKASHI Takahiro <takahiro.akashi@...aro.org>
To:     Julien Thierry <julien.thierry@....com>
Cc:     catalin.marinas@....com, will.deacon@....com,
        bauerman@...ux.vnet.ibm.com, dhowells@...hat.com,
        vgoyal@...hat.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, akpm@...ux-foundation.org, mpe@...erman.id.au,
        dyoung@...hat.com, bhe@...hat.com, arnd@...db.de,
        ard.biesheuvel@...aro.org, kexec@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 03/10] kexec_file: factor out arch_kexec_kernel_*()
 from x86, powerpc

On Wed, Oct 11, 2017 at 09:24:16AM +0100, Julien Thierry wrote:
> 
> 
> On 11/10/17 06:07, AKASHI Takahiro wrote:
> >On Tue, Oct 10, 2017 at 12:02:01PM +0100, Julien Thierry wrote:
> >
> >[snip]
> >
> >>>--- a/kernel/kexec_file.c
> >>>+++ b/kernel/kexec_file.c
> >>>@@ -26,30 +26,79 @@
> >>>  #include <linux/vmalloc.h>
> >>>  #include "kexec_internal.h"
> >>>
> >>>+const __weak struct kexec_file_ops * const kexec_file_loaders[] = {NULL};
> >>>+
> >>>  static int kexec_calculate_store_digests(struct kimage *image);
> >>>
> >>>+int _kexec_kernel_image_probe(struct kimage *image, void *buf,
> >>>+                          unsigned long buf_len)
> >>>+{
> >>>+     const struct kexec_file_ops *fops;
> >>>+     int ret = -ENOEXEC;
> >>>+
> >>>+     for (fops = kexec_file_loaders[0]; fops && fops->probe; ++fops) {
> >>
> >>Hmm, that's not gonna work (and I see that what I said in the previous
> >>patch was not 100% correct either).
> >
> >Can you elaborate this a bit more?
> >
> 
> Yes. With the current state of the loop, you are going to check the first
> element of kexec_file_loaders[0], and what will get incremented is the
> pointer contained in kexec_file_loaders rather than a pointer pointer
> pointing at an element of kexec_file_loaders.

Aha, got it. I thought that you were talking about const usage.

Since I don't want to bother anybody with my repeated minor updates,
I'd like to post a new version, v6, only if you don't have any more
comments and if Catalin and Will are likely to accept my other patches.

Thanks,
-Takahiro AKASHI


> 
> >I'm sure that, with my code, any member of fops, cannot be changed;
> >"const struct kexec_file_ops *fops" means that fops is a pointer to
> >"constant sturct kexec_file_ops," while "struct kexec_file_ops *
> >const kexec_file_loaders[]" means that kexec_file_loaders is a "constant
> >array" of pointers to "constant struct kexec_file_ops."
> >
> 
> Hmm, right, my suggestion below doesn't have the right constness, fops
> should be declared as:
> const struct kexec_file_ops * const * fops;
> 
> This can point at elements of kexec_file_loaders.
> 
> Hope this makes more sense.
> 
> Cheers,
> 
> >Thanks,
> >-Takahiro AKASHI
> >
> >
> >>'fops' should be of type 'const struct kexec_file_ops **', and the loop
> >>should be:
> >>
> >>for (fops = &kexec_file_loaders[0]; *fops && (*fops)->probe; ++fops)
> >>
> >>With some additional dereferences in the body of the loop.
> >>
> >>Unless you prefer the previous state of the loop (with i and the break
> >>inside), but I still think this looks better.
> >>
> >>Cheers,
> >>
> 
> 
> -- 
> Julien Thierry