musl - [PATCH] Possible patch for __syscall

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Message-ID: <DB6PR0502MB3016ADDF1A67C6E0264F9B7DE7F60@DB6PR0502MB3016.eurprd05.prod.outlook.com>
Date: Tue, 13 Feb 2018 14:28:32 +0000
From: Nicholas Wilson <nicholas.wilson@...lvnc.com>
To: "musl@...ts.openwall.com" <musl@...ts.openwall.com>
Subject: [PATCH] Possible patch for __syscall_cp

Hi,

I have here a patch, which may or not be correct - I'm not confident.

In __syscall_cp.c, there's a call to __syscall which deliberately *disables* macro expansion, forcing the call to go via the __syscall function. On Wasm, I don't currently provide this function, instead I'm using the macros for __syscall to redirect via the __syscall<N> functions.

This call in __syscall_cp looks to be practically the only place in the whole of Musl where macro expansion is prevented for __syscall (other than in src/internal/syscall.h itself, and arch/mips/syscall_arch.h). So on those grounds it's a bit suspicious, given that every other caller of __syscall uses the macros from internal/syscall.h.

If there is a rationale, I could just define __syscall() in arch/wasm/syscall_arch.h - but I'd rather not if it's not meant to be called. Would it be possible instead to apply the patch below, and allow macro expansion?

I can't see a risk, although I don't understand the cancellation-point implementation very well. There's no recursion possible, since plain __syscall() never redirects to any of the __syscall_cp machinery, so expansion oughtn't to cause problems on any archs?

Thanks,
Nick

diff --git a/src/thread/__syscall_cp.c b/src/thread/__syscall_cp.c
index 09a2be84..7b870faa 100644
--- a/src/thread/__syscall_cp.c
+++ b/src/thread/__syscall_cp.c
@@ -8,7 +8,7 @@ static long sccp(syscall_arg_t nr,
                  syscall_arg_t u, syscall_arg_t v, syscall_arg_t w,
                  syscall_arg_t x, syscall_arg_t y, syscall_arg_t z)
 {
-       return (__syscall)(nr, u, v, w, x, y, z);
+       return __syscall(nr, u, v, w, x, y, z);
 }

 weak_alias(sccp, __syscall_cp_c);

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