Subject: [RFC PATCH 1/2] powerpc/64: remove system call instruction emulation

From: Nicholas Piggin <[email-masked]>
Date: Tue, 18 Jan 2022 15:58:20 +1000

emulate_step instruction emulation including sc instruction emulation
initially appeared in xmon. It then emulation code was then moved into
sstep.c where kprobes could use it too, and later hw_breakpoint and
uprobes started to use it.

Until uprobes, the only instruction emulation users were for kernel
mode instructions.

- xmon only steps / breaks on kernel addresses.
- kprobes is kernel only.
- hw_breakpoint only emulates kernel instructions, single steps user.

At one point there was support for the kernel to execute sc
instructions, although that is long removed and it's not clear whether
there was any upstream instructions or this was used by out of tree
modules. So system call emulation is not required by the above users.

uprobes uses emulate_step and it appears possible to emulate sc
instruction in userspace. This type of system call emulation is broken
and it's not clear it ever worked well.

The big complication is that userspace takes an interrupt to the kernel
to emulate the instruction. The user->kernel interrupt sets up registers
and interrupt stack frame expecting to return to userspace, then system
call instruction emulation re-directs that stack frame to the kernel,
early in the system call interrupt handler. This means the the interrupt
return code takes the kernel->kernel restore path, which does not restore
everything as the system call interrupt handler would expect coming from
userspace. regs->iamr appears to get lost for example, because the
kernel->kernel return does not restore the user iamr. Accounting such as
irqflags tracing and CPU accounting does not get flipped back to user
mode as the system call handler expects, so those appear to enter the
kernel twice without returning to userspace.

These things may be individually fixable with various complication, but
it is a big complexity to support this just for uprobes.

uprobes emulates the instruction rather than stepping for performance
reasons. System calls instructions should not be a significant source
of uprobes and already expensive, so skipping emulation should not be
a significant problem.

Signed-off-by: Nicholas Piggin <[email-masked]>
---
 arch/powerpc/lib/sstep.c | 36 ------------------------------------
 1 file changed, 36 deletions(-)

diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
index a94b0cd0bdc5..ee3bc45fb23b 100644
--- a/arch/powerpc/lib/sstep.c
+++ b/arch/powerpc/lib/sstep.c
@@ -15,9 +15,6 @@
 #include <asm/cputable.h>
 #include <asm/disassemble.h>
 
-extern char system_call_common[];
-extern char system_call_vectored_emulate[];
-
 #ifdef CONFIG_PPC64
 /* Bits in SRR1 that are copied from MSR */
 #define MSR_MASK	0xffffffff87c0ffffUL
@@ -3650,39 +3647,6 @@ int emulate_step(struct pt_regs *regs, ppc_inst_t instr)
 		goto instr_done;
 
 #ifdef CONFIG_PPC64
-	case SYSCALL:	/* sc */
-		/*
-		 * N.B. this uses knowledge about how the syscall
-		 * entry code works.  If that is changed, this will
-		 * need to be changed also.
-		 */
-		if (IS_ENABLED(CONFIG_PPC_FAST_ENDIAN_SWITCH) &&
-				cpu_has_feature(CPU_FTR_REAL_LE) &&
-				regs->gpr[0] == 0x1ebe) {
-			regs_set_return_msr(regs, regs->msr ^ MSR_LE);
-			goto instr_done;
-		}
-		regs->gpr[9] = regs->gpr[13];
-		regs->gpr[10] = MSR_KERNEL;
-		regs->gpr[11] = regs->nip + 4;
-		regs->gpr[12] = regs->msr & MSR_MASK;
-		regs->gpr[13] = (unsigned long) get_paca();
-		regs_set_return_ip(regs, (unsigned long) &system_call_common);
-		regs_set_return_msr(regs, MSR_KERNEL);
-		return 1;
-
-#ifdef CONFIG_PPC_BOOK3S_64
-	case SYSCALL_VECTORED_0:	/* scv 0 */
-		regs->gpr[9] = regs->gpr[13];
-		regs->gpr[10] = MSR_KERNEL;
-		regs->gpr[11] = regs->nip + 4;
-		regs->gpr[12] = regs->msr & MSR_MASK;
-		regs->gpr[13] = (unsigned long) get_paca();
-		regs_set_return_ip(regs, (unsigned long) &system_call_vectored_emulate);
-		regs_set_return_msr(regs, MSR_KERNEL);
-		return 1;
-#endif
-
 	case RFI:
 		return -1;
 #endif
-- 
2.23.0