1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
|
From d078d0aa86e9e3b937f673dc89306b3afd09d560 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= <roger.pau@citrix.com>
Date: Thu, 25 Jul 2024 16:21:17 +0200
Subject: [PATCH 56/56] x86/altcall: fix clang code-gen when using altcall in
loop constructs
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Yet another clang code generation issue when using altcalls.
The issue this time is with using loop constructs around alternative_{,v}call
instances using parameter types smaller than the register size.
Given the following example code:
static void bar(bool b)
{
unsigned int i;
for ( i = 0; i < 10; i++ )
{
int ret_;
register union {
bool e;
unsigned long r;
} di asm("rdi") = { .e = b };
register unsigned long si asm("rsi");
register unsigned long dx asm("rdx");
register unsigned long cx asm("rcx");
register unsigned long r8 asm("r8");
register unsigned long r9 asm("r9");
register unsigned long r10 asm("r10");
register unsigned long r11 asm("r11");
asm volatile ( "call %c[addr]"
: "+r" (di), "=r" (si), "=r" (dx),
"=r" (cx), "=r" (r8), "=r" (r9),
"=r" (r10), "=r" (r11), "=a" (ret_)
: [addr] "i" (&(func)), "g" (func)
: "memory" );
}
}
See: https://godbolt.org/z/qvxMGd84q
Clang will generate machine code that only resets the low 8 bits of %rdi
between loop calls, leaving the rest of the register possibly containing
garbage from the use of %rdi inside the called function. Note also that clang
doesn't truncate the input parameters at the callee, thus breaking the psABI.
Fix this by turning the `e` element in the anonymous union into an array that
consumes the same space as an unsigned long, as this forces clang to reset the
whole %rdi register instead of just the low 8 bits.
Fixes: 2ce562b2a413 ('x86/altcall: use a union as register type for function parameters on clang')
Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: d51b2f5ea1915fe058f730b0ec542cf84254fca0
master date: 2024-07-23 13:59:30 +0200
---
xen/arch/x86/include/asm/alternative.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/xen/arch/x86/include/asm/alternative.h b/xen/arch/x86/include/asm/alternative.h
index 0d3697f1de..e63b459276 100644
--- a/xen/arch/x86/include/asm/alternative.h
+++ b/xen/arch/x86/include/asm/alternative.h
@@ -185,10 +185,10 @@ extern void alternative_branches(void);
*/
#define ALT_CALL_ARG(arg, n) \
register union { \
- typeof(arg) e; \
+ typeof(arg) e[sizeof(long) / sizeof(arg)]; \
unsigned long r; \
} a ## n ## _ asm ( ALT_CALL_arg ## n ) = { \
- .e = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); }) \
+ .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })\
}
#else
#define ALT_CALL_ARG(arg, n) \
--
2.45.2
|
GitWeb