From: Willy Tarreau w@1wt.eu
[ 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@1wt.eu Signed-off-by: Paul E. McKenney paulmck@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- tools/include/nolibc/arch-mips.h | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h index 1a124790c99f..5d647afa42e6 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