From: Peter Zijlstra peterz@infradead.org
commit 1cd5f059d956e6f614ba6666ecdbcf95db05d5f5 upstream.
Paolo reported that the instruction sequence that is used to replace:
call __static_call_return0
namely:
66 66 48 31 c0 data16 data16 xor %rax,%rax
decodes to something else on i386, namely:
66 66 48 data16 dec %ax 31 c0 xor %eax,%eax
Which is a nonsensical sequence that happens to have the same outcome. *However* an important distinction is that it consists of 2 instructions which is a problem when the thing needs to be overwriten with a regular call instruction again.
As such, replace the instruction with something that decodes the same on both i386 and x86_64.
Fixes: 3f2a8fc4b15d ("static_call/x86: Add __static_call_return0()") Reported-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lkml.kernel.org/r/20220318204419.GT8939@worktop.programming.kicks-as... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kernel/static_call.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)
--- a/arch/x86/kernel/static_call.c +++ b/arch/x86/kernel/static_call.c @@ -12,10 +12,9 @@ enum insn_type { };
/* - * data16 data16 xorq %rax, %rax - a single 5 byte instruction that clears %rax - * The REX.W cancels the effect of any data16. + * cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax */ -static const u8 xor5rax[] = { 0x66, 0x66, 0x48, 0x31, 0xc0 }; +static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };