From e3848e3dd64a8d6437531488fe341354bc02eaed Mon Sep 17 00:00:00 2001 From: Lucian Cojocar Date: Fri, 10 Jun 2016 18:44:44 +0200 Subject: bugfix: ARM: memset.S: use unsigned comparisons The 'BLT' instruction checks for *signed* values. So if a3, length parameter of memset, is negative, then value added to the PC will be large. memset(buf, 0xaa, 0xffff0000) triggers the bug. GDB session without the patch: """ $ gdb ./main-buggy-memset.elf -q Reading symbols from ./main-buggy-memset.elf...done. (gdb) x/i memset 0x8770 : mov r3, r0 (gdb) r Starting program: /root/memset/main-buggy-memset.elf Program received signal SIGSEGV, Segmentation fault. 0x00048808 in ?? () """ The $pc is outside of the memset function because: """ (gdb) x/i $pc => 0x87e4 : add pc, pc, r2, lsl #2 (gdb) info reg $r2 r2 0x10007 65543 """ GDB session with the bug fixed (patch applied): """ $ gdb ./main-fixed-memset.elf -q Reading symbols from ./main-fixed-memset.elf...done. (gdb) x/i memset 0x8770 : mov r3, r0 (gdb) r Starting program: /root/memset/main-fixed-memset.elf Program received signal SIGSEGV, Segmentation fault. memset () at libc/string/arm/memset.S:92 92 libc/string/arm/memset.S: No such file or directory. (gdb) x/i $pc => 0x87b0 : stmia r3!, {r1, r12} (gdb) info reg $r3 r3 0x15000 86016 (gdb) info proc mappings process 5822 Mapped address spaces: Start Addr End Addr Size Offset objfile 0x8000 0xb000 0x3000 0x0 /root/memset/main-fixed-memset.elf 0x12000 0x15000 0x3000 0x2000 /root/memset/main-fixed-memset.elf 0xb6fff000 0xb7000000 0x1000 0x0 [sigpage] 0xbefdf000 0xbf000000 0x21000 0x0 0xffff0000 0xffff1000 0x1000 0x0 [vectors] (gdb) info reg $sp sp 0x14d78 0x14d78 """ GDB crashes inside the memset function, on the store instruction. This time the crash is (as expected) because of a memory access imediately after the memory region that contains the stack -- the buffer that's being memset'd is allocated on the stack. Signed-off-by: Lucian Cojocar --- libc/string/arm/memset.S | 20 ++++++++++---------- 1 file changed, 10 insertions(+), 10 deletions(-) (limited to 'libc') diff --git a/libc/string/arm/memset.S b/libc/string/arm/memset.S index 2be4850e4..412270f50 100644 --- a/libc/string/arm/memset.S +++ b/libc/string/arm/memset.S @@ -67,7 +67,7 @@ memset: memset: mov a4, a1 cmp a3, $8 @ at least 8 bytes to do? - blt 2f + blo 2f orr a2, a2, a2, lsl $8 orr a2, a2, a2, lsl $16 1: @@ -84,27 +84,27 @@ memset: mov ip, a2 1: cmp a3, $8 @ 8 bytes still to do? - blt 2f + blo 2f stmia a4!, {a2, ip} sub a3, a3, $8 cmp a3, $8 @ 8 bytes still to do? - blt 2f + blo 2f stmia a4!, {a2, ip} sub a3, a3, $8 cmp a3, $8 @ 8 bytes still to do? - blt 2f + blo 2f stmia a4!, {a2, ip} sub a3, a3, $8 cmp a3, $8 @ 8 bytes still to do? #if defined(__thumb2__) - itt ge - stmiage a4!, {a2, ip} - subge a3, a3, $8 + itt hs + stmiahs a4!, {a2, ip} + subhs a3, a3, $8 #else - stmgeia a4!, {a2, ip} - subge a3, a3, $8 + stmhsia a4!, {a2, ip} + subhs a3, a3, $8 #endif - bge 1b + bhs 1b 2: movs a3, a3 @ anything left? IT(t, eq) -- cgit v1.2.3