This is a note to let you know that I've just added the patch titled
tools/nolibc: restore mips branch ordering in the _start block
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
tools-nolibc-restore-mips-branch-ordering-in-the-_st.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 74c6ada39b016d83095dea69585ec5455f9e9fff
Author: Willy Tarreau <w@xxxxxx>
Date: Mon Jan 9 08:54:39 2023 +0100
tools/nolibc: restore mips branch ordering in the _start block
[ Upstream commit 184177c3d6e023da934761e198c281344d7dd65b ]
Depending on the compiler used and the optimization options, the sbrk()
test was crashing, both on real hardware (mips-24kc) and in qemu. One
such example is kernel.org toolchain in version 11.3 optimizing at -Os.
Inspecting the sys_brk() call shows the following code:
0040047c <sys_brk>:
40047c: 24020fcd li v0,4045
400480: 27bdffe0 addiu sp,sp,-32
400484: 0000000c syscall
400488: 27bd0020 addiu sp,sp,32
40048c: 10e00001 beqz a3,400494 <sys_brk+0x18>
400490: 00021023 negu v0,v0
400494: 03e00008 jr ra
It is obviously wrong, the "negu" instruction is placed in beqz's
delayed slot, and worse, there's no nop nor instruction after the
return, so the next function's first instruction (addiu sip,sip,-32)
will also be executed as part of the delayed slot that follows the
return.
This is caused by the ".set noreorder" directive in the _start block,
that applies to the whole program. The compiler emits code without the
delayed slots and relies on the compiler to swap instructions when this
option is not set. Removing the option would require to change the
startup code in a way that wouldn't make it look like the resulting
code, which would not be easy to debug. Instead let's just save the
default ordering before changing it, and restore it at the end of the
_start block. Now the code is correct:
0040047c <sys_brk>:
40047c: 24020fcd li v0,4045
400480: 27bdffe0 addiu sp,sp,-32
400484: 0000000c syscall
400488: 10e00002 beqz a3,400494 <sys_brk+0x18>
40048c: 27bd0020 addiu sp,sp,32
400490: 00021023 negu v0,v0
400494: 03e00008 jr ra
400498: 00000000 nop
Fixes: 66b6f755ad45 ("rcutorture: Import a copy of nolibc") #5.0
Signed-off-by: Willy Tarreau <w@xxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h
index 5fc5b8029bff..7380093ba9e7 100644
--- a/tools/include/nolibc/arch-mips.h
+++ b/tools/include/nolibc/arch-mips.h
@@ -192,6 +192,7 @@ struct sys_stat_struct {
__asm__ (".section .text\n"
".weak __start\n"
".set nomips16\n"
+ ".set push\n"
".set noreorder\n"
".option pic0\n"
".ent __start\n"
@@ -210,6 +211,7 @@ __asm__ (".section .text\n"
"li $v0, 4001\n" // NR_exit == 4001
"syscall\n"
".end __start\n"
+ ".set pop\n"
"");
#endif // _NOLIBC_ARCH_MIPS_H
