[PATCH 1/2] arm64: syscallno is secretly an int, make it official

On Tue, Aug 01, 2017 at 12:36:47PM +0100, Will Deacon wrote:

On Tue, Jul 18, 2017 at 01:41:43PM +0100, Dave Martin wrote:

quoted

The upper 32 bits of the syscallno field in struct pt_regs are handled
inconsistently, being sometimes zero extended and sometimes
sign-extended.  The reason for this appears to be that they are not
intentionally significant at all.

Currently, the only place I can find where those bits are
significant is in calling trace_sys_enter(), which may be
unintentional: for example, if a compat tracer attempts to cancel a
syscall by passing -1 to (COMPAT_)PTRACE_SET_SYSCALL at the
syscall-enter-stop, it will be traced as syscall 4294967295
rather than -1 as might be expected (and as occurs for a native
tracer doing the same thing).  Elsewhere, reads of syscallno cast
it to an int or truncate it.

There's also a conspicuous amount of code and casting to bodge
around the fact that although semantically an int, syscallno is
stored as a u64.

Let's not pretend any more.

In order to preserve the stp x instruction that stores the syscall
number in entry.S, this patch special-cases the layout of struct
pt_regs for big endian so that the newly 32-bit syscallno field
maps onto the low bits of the stored value.  This is not beautiful,
but benchmarking of the getpid syscall on Juno suggests indicates a
minor slowdown if the stp is split into an stp x and stp w.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>

[...]

quoted

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index 11403fd..21c87dc 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h

@@ -116,7 +116,14 @@ struct pt_regs {
 		};
 	};
 	u64 orig_x0;
-	u64 syscallno;
+#ifdef __AARCH64EB__
+	u32 unused2;
+	s32 syscallno;
+#else
+	s32 syscallno;
+	u32 unused2;
+#endif
+
 	u64 orig_addr_limit;
 	u64 unused;	// maintain 16 byte alignment

This isn't a UAPI struct, so we could juggle these about a bit to put all
the unused padding at the end.

In the __AARCH64EB__ case syscallno still needs to be padded up to an
odd 32-bit boundary, so we need padding before it in that case; also
we'd still need the #ifdef either way, and the struct is already
intentionally a multiple of 16 bytes to align it on the stack.  So I'm
not sure what we'd save here (?)

I could put orig_addr_limit before (padded) syscallno, but I'm not
sure that's better.

Cheers
---Dave