On older versions of binutils, \sym points to an aligned address. On newer versions of binutils, \sym sometimes points to the unaligned thumb address in mysterious and buggy circumstances. In order to homogenize this behavior, rather than adding 1, we simply OR in 1, so that already unaligned instructions don't change. This fix is required for a pedestrian THUMB2_KERNEL to boot without crashing when built with non-old binutils.
While it works, the downside is that we have to add an `orr` instruction to a fast path. The assembler can't do this at assemble time via "|1" because "invalid operands (.text and *ABS* sections) for `|'", so we're forced to do this. A better solution would be to have consistent binutils behavior, or to have some kind of \sym feature detection that won't turn into a maze of version comparisons. However, it's at the moment unclear how to achieve this.
The rest of this commit message contains all of the relevant information.
My tests concerned these versions: broken: GNU ld (Gentoo 2.29.1 p3) 2.29.1 working: GNU ld (GNU Binutils for Ubuntu) 2.26.1
These produced the following code: --- broken 2017-11-21 17:44:14.523416082 +0100 +++ working 2017-11-21 17:44:44.548461234 +0100 @@ -133,7 +133,7 @@ 160: f01a 0ff0 tst.w sl, #240 ; 0xf0 164: d111 bne.n 18a <__sys_trace> 166: f5b7 7fc8 cmp.w r7, #400 ; 0x190 - 16a: f2af 1e6a subw lr, pc, #362 ; 0x16a + 16a: f2af 1e6b subw lr, pc, #363 ; 0x16b 16e: bf38 it cc 170: f858 f027 ldrcc.w pc, [r8, r7, lsl #2] 174: a902 add r1, sp, #8
The differing instruction corresponds with this actual line in arch/arm/kernel/entry-common.S: badr lr, ret_fast_syscall @ return address
Running the broken kernel results in a runtime OOPS with: PC is at ret_fast_syscall+0x4/0x52 LR is at ret_fast_syscall+0x2/0x52
The disassembly of that function for the crashing kernel is: .text:00000000 ret_fast_syscall ; CODE XREF: sys_syscall+1C↓j .text:00000000 CPSID I ; jumptable 00000840 cases 15,18-376 .text:00000002 .text:00000002 loc_2 ; DATA XREF: sys_syscall-6BA↓o .text:00000002 LDR.W R2, [R9,#8] .text:00000006 CMP.W R2, #0xBF000000
Signed-off-by: Jason A. Donenfeld Jason@zx2c4.com Cc: stable@vger.kernel.org --- arch/arm/include/asm/assembler.h | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h index ad301f107dd2..c62a3b6b0a3e 100644 --- a/arch/arm/include/asm/assembler.h +++ b/arch/arm/include/asm/assembler.h @@ -194,10 +194,9 @@ */ .irp c,,eq,ne,cs,cc,mi,pl,vs,vc,hi,ls,ge,lt,gt,le,hs,lo .macro badr\c, rd, sym -#ifdef CONFIG_THUMB2_KERNEL - adr\c \rd, \sym + 1 -#else adr\c \rd, \sym +#ifdef CONFIG_THUMB2_KERNEL + orr\c \rd, \rd, 1 #endif .endm .endr