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From: Paul Burton <paulburton@kernel.org>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Christian Brauner <christian.brauner@canonical.com>
Subject: Re: [PATCH] mips: vdso: conditionalize 32-bit time functions on COMPAT_32BIT_TIME
Date: Tue, 31 Dec 2019 20:10:56 -0800	[thread overview]
Message-ID: <20200101041056.xionott76qbrffih@lantea.localdomain> (raw)
In-Reply-To: <CAHmME9q0z4RBhy7AETo0sPt+SfV7=zFra0iyZ-Sm54c-WG6CVg@mail.gmail.com>

Hi Jason,

On Tue, Dec 31, 2019 at 05:14:41PM +0100, Jason A. Donenfeld wrote:
> Here's a "one click" reproducer:
> https://data.zx2c4.com/mips-musl-libc-weird-crash-time32-compat.tar.xz
> 
> Untar that and hit `make -j$(nproc)`, and you'll get a freshly built
> and crashing kernel+userland.

Thanks for the test case. It seems like the VDSO code isn't saving &
restoring $gp/$28, even though it's meant to be callee-saved in both the
n32 & n64 ABIs. With some digging I found that the below seems to
resolve the issue. Could you check whether it works for you?

I'm still not quite sure *why* this happens; perhaps GCC just decides it
doesn't need to save & restore $gp/$28 when it spots that it's being
"used" for __current_thread_info (even though that's never actually
referenced in the VDSO)?

Just moving the declaration of __current_thread_info inside the
current_thread_info() function seems to do the trick too, and is
probably a bit neater.

Thanks,
    Paul

---
diff --git a/arch/mips/include/asm/thread_info.h b/arch/mips/include/asm/thread_info.h
index 4993db40482c..ac33959bbb1f 100644
--- a/arch/mips/include/asm/thread_info.h
+++ b/arch/mips/include/asm/thread_info.h
@@ -50,7 +50,11 @@ struct thread_info {
 }

 /* How to get the thread information struct from C.  */
+#ifdef __VDSO__
+register struct thread_info *__current_thread_info __asm__("$0");
+#else
 register struct thread_info *__current_thread_info __asm__("$28");
+#endif

 static inline struct thread_info *current_thread_info(void)
 {

  reply	other threads:[~2020-01-01  4:08 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-23 13:08 vdso-related userspace crashes on 5.5 mips64 Jason A. Donenfeld
2019-12-23 21:44 ` Jason A. Donenfeld
2019-12-23 23:29 ` Paul Burton
2019-12-24 13:37   ` Jason A. Donenfeld
2019-12-30 15:58     ` Arnd Bergmann
2019-12-24 14:19   ` Jason A. Donenfeld
2019-12-24 13:54 ` [PATCH] mips: vdso: conditionalize 32-bit time functions on COMPAT_32BIT_TIME Jason A. Donenfeld
2019-12-30 11:57   ` Arnd Bergmann
2019-12-30 12:26     ` Jason A. Donenfeld
2019-12-30 12:34       ` Arnd Bergmann
2019-12-30 14:37         ` Jason A. Donenfeld
2019-12-30 15:10           ` Jason A. Donenfeld
2019-12-30 15:37           ` Arnd Bergmann
2019-12-30 15:39             ` Jason A. Donenfeld
2019-12-30 15:47               ` Arnd Bergmann
2019-12-30 15:58                 ` Jason A. Donenfeld
2019-12-30 17:33                   ` Arnd Bergmann
2019-12-30 21:09                     ` Jason A. Donenfeld
2019-12-30 21:42                       ` Jason A. Donenfeld
2019-12-31 16:14                         ` Jason A. Donenfeld
2020-01-01  4:10                           ` Paul Burton [this message]
2020-01-01  4:25                             ` Paul Burton
2020-01-01  9:47                               ` Jason A. Donenfeld
2020-01-01  9:47                             ` Jason A. Donenfeld

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