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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Balbir Singh <bsingharora@gmail.com>, erhard_f@mailbox.org,
        jack@suse.cz, linuxppc-dev@ozlabs.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        aneesh.kumar@linux.vnet.ibm.com
Subject: Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present()
In-Reply-To: <20190219201539.GT14180@gate.crashing.org>
References: <20190214062339.7139-1-mpe@ellerman.id.au> <20190216105511.GA31125@350D> <20190216142206.GE14180@gate.crashing.org> <20190217062333.GC31125@350D> <87ef86dd9v.fsf@concordia.ellerman.id.au> <20190217215556.GH31125@350D> <87imxhrkdt.fsf@concordia.ellerman.id.au> <20190219201539.GT14180@gate.crashing.org>
Date:   Wed, 20 Feb 2019 22:18:38 +1100
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Segher Boessenkool <segher@kernel.crashing.org> writes:
> On Mon, Feb 18, 2019 at 11:49:18AM +1100, Michael Ellerman wrote:
>> Balbir Singh <bsingharora@gmail.com> writes:
>> > Fair enough, my point was that the compiler can help out. I'll see what
>> > -Wconversion finds on my local build :)
>> 
>> I get about 43MB of warnings here :)
>
> Yes, -Wconversion complains about a lot of things that are idiomatic C.
> There is a reason -Wconversion is not in -Wall or -Wextra.

Actually a lot of those go away when I add -Wno-sign-conversion.

And what's left seems mostly reasonable, they all indicate the
possibility of a bug I think.

In fact this works and would have caught the bug:

diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
index d8c8d7c9df15..3114e3f368e2 100644
--- a/arch/powerpc/include/asm/book3s/64/pgtable.h
+++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
@@ -904,7 +904,12 @@ static inline int pud_none(pud_t pud)
 
 static inline int pud_present(pud_t pud)
 {
+	__diag_push();
+	__diag_warn(GCC, 8, "-Wconversion", "ulong -> int");
+
 	return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PRESENT));
+
+	__diag_pop();
 }
 
 extern struct page *pud_page(pud_t pud);


Obviously we're not going to instrument every function like that. But we
could start instrumenting particular files.

cheers