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Message-ID: <9d7873b6-e35c-ae38-9952-a1df443b2aea@redhat.com>
Date: Wed, 16 Jun 2021 09:22:02 -0700
From: Tom Rix <trix@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Zheng Zengkai <zhengzengkai@...wei.com>,
Randy Dunlap <rdunlap@...radead.org>,
David Howells <dhowells@...hat.com>,
Hulk Robot <hulkci@...wei.com>, linux-afs@...ts.infradead.org,
Marc Dionne <marc.dionne@...istor.com>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] afs: fix no return statement in function returning
non-void
On 6/16/21 7:34 AM, Linus Torvalds wrote:
> On Wed, Jun 16, 2021 at 5:56 AM Tom Rix <trix@...hat.com> wrote:
>> A fix is to use the __noreturn attribute on this function
> That's certainly a better thing. It would be better yet to figure out
> why BUG() didn't do it automatically.
>
> Without CONFIG_BUG, it looks like powerpc picks up
>
> #ifndef HAVE_ARCH_BUG
> #define BUG() do {} while (1)
>
> which should still make it pointless to have the return. But I might
> have missed something.
This looks like a problem the generic BUG().
with CONFIG_BUG=y, the *.i is
static int afs_dir_set_page_dirty(struct page *page)
{
do { __asm__ __volatile__( "1: " "twi 31, 0, 0" "\n" ".section
__bug_table,\"aw\"\n" "2:\t.4byte 1b - 2b, %0 - 2b\n" "\t.short %1,
%2\n" ".org 2b+%3\n" ".previous\n" : : "i" ("fs/afs/dir.c"), "i" (50),
"i" (0), "i" (sizeof(struct bug_entry))); do { ; asm volatile("");
__builtin_unreachable(); } while (0); } while (0);
}
BUG() expanded from
#define BUG() do { \
BUG_ENTRY("twi 31, 0, 0", 0); \
unreachable(); \
} while (0)
with CONFIG_BUG=n, the *.i is
static int afs_dir_set_page_dirty(struct page *page)
{
do {} while (1);
}
BUG() expanded from
do {} while (1)
to fix, add an unreachable() to the generic BUG()
diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index f152b9bb916f..b250e06d7de2 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -177,7 +177,10 @@ void __warn(const char *file, int line, void
*caller, unsigned taint,
#else /* !CONFIG_BUG */
#ifndef HAVE_ARCH_BUG
-#define BUG() do {} while (1)
+#define BUG() do { \
+ do {} while (1); \
+ unreachable(); \
+ } while (0)
#endif
the new *.i
static int afs_dir_set_page_dirty(struct page *page)
{
do { do {} while (1); do { ; asm volatile("");
__builtin_unreachable(); } while (0); } while (0);
}
The assembly is unchanged.
The key being the unreachable builtin
ref: gcc docs https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
" ... without the __builtin_unreachable, GCC issues a
warning that control reaches the end of a non-void function."
Tom
>
> Linus
>
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