On Sat, Jul 15, 2017 at 12:10:58AM -0500, Josh Poimboeuf wrote:
> Mike Galbraith reported a situation where a WARN_ON_ONCE() call in DRM
> code turned into an oops. As it turns out, WARN_ON_ONCE() seems to be
> completely broken when called from a module.
>
> The bug was introduced with the following commit:
>
> 19d436268dde ("debug: Add _ONCE() logic to report_bug()")
>
> That commit changed WARN_ON_ONCE() to move its 'once' logic into the bug
> trap handler. It requires a writable bug table so that the BUGFLAG_DONE
> bit can be written to the flags to indicate the first warning has
> occurred.
>
> The bug table was made writable for vmlinux, which relies on
> vmlinux.lds.S and vmlinux.lds.h for laying out the sections. However,
> it wasn't made writable for modules, which rely on the ELF section
> header flags.
>
> Reported-by: Mike Galbraith <[email protected]>
> Fixes: 19d436268dde ("debug: Add _ONCE() logic to report_bug()")
> Signed-off-by: Josh Poimboeuf <[email protected]>
> ---
> diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> index 39e702d..aa6b202 100644
> --- a/arch/x86/include/asm/bug.h
> +++ b/arch/x86/include/asm/bug.h
> @@ -35,7 +35,7 @@
> #define _BUG_FLAGS(ins, flags)
> \
> do { \
> asm volatile("1:\t" ins "\n" \
> - ".pushsection __bug_table,\"a\"\n" \
> + ".pushsection __bug_table,\"aw\"\n" \
> "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n" \
> "\t" __BUG_REL(%c0) "\t# bug_entry::file\n" \
> "\t.word %c1" "\t# bug_entry::line\n" \
> @@ -52,7 +52,7 @@ do {
> \
> #define _BUG_FLAGS(ins, flags)
> \
> do { \
> asm volatile("1:\t" ins "\n" \
> - ".pushsection __bug_table,\"a\"\n" \
> + ".pushsection __bug_table,\"aw\"\n" \
> "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n" \
> "\t.word %c0" "\t# bug_entry::flags\n" \
> "\t.org 2b+%c1\n" \
Ah shiny. Thanks Josh!
