Commit 503b03daa30bfcebbd45f27feff87f74b11b45a6 | zig

author	Geyslan Gregório <gg@condado.dev> 2026-10-14 20:53:01-03:00
committer	Alex Rønne Petersen <alex@alexrp.com> 2026-10-15 19:33:50+02:00
log	503b03daa30bfcebbd45f27feff87f74b11b45a6
tree	a18e7aa51632de8924d2011a1c338dbc36560993
parent	95d4703bee77ead0bc21f3cf8ca4c09edeba6bd4

std.os.linux: declare $a3 as an output of mips syscall4 and up

On MIPS, $a3 ($7) carries the fourth argument [1] and returns the
kernel's error flag [2]. The kernel writes the flag on every return,
success included [3]. syscall4 and up declared $7 an input only, so
the compiler assumed it still held the argument after the syscall.
In release builds, a pointer passed as the fourth argument then reads
back as the flag, 0 on success, and using it faults at address 0.

Commit 5b29b4ffa6 ("Fix MIPS inline assembly clobbers") took $7 out of
the clobbers, since an input cannot be one, but never made it an
output.

Add that output, as alpha.zig does for its argument registers [4]
and musl does with "+r" [5]. $7 stays out of the clobbers, because a
register that is input, output and clobber at once can miscompile (#35724).

[1] - https://refspecs.linuxfoundation.org/elf/mipsabi.pdf#page=24
[2] - https://man7.org/linux/man-pages/man2/syscall.2.html
[3] - https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/mips/kernel/scall32-o32.S?h=v7.1#n112
[4] - https://codeberg.org/ziglang/zig/src/commit/d8938120130dbe48ee8764b502b18c6224ab9340/lib/std/os/linux/alpha.zig#L38-L46
[5] - https://git.musl-libc.org/cgit/musl/tree/arch/mips/syscall_arch.h?h=v1.2.6#n81

closes #37247

3 files changed, 30 insertions(+), 0 deletions(-)

lib
- std
  - os
    linux
    mips.zig
    mips64.zig
    mipsn32.zig

lib/std/os/linux/mips.zig+12

...	...	@@ -75,12 +75,15 @@ pub fn syscall4(
75	75	arg3: syscall_arg_t,
76	76	arg4: syscall_arg_t,
77	77	) u32 {
	78	// $7 is both an input and a clobber.
	79	var r7_out: syscall_arg_t = undefined;
78	80	return asm volatile (
79	81	\\ syscall
80	82	\\ beq $a3, $zero, 1f
81	83	\\ subu $v0, $zero, $v0
82	84	\\1:
83	85	: [ret] "={$2}" (-> u32),
	86	[r7_out] "={$7}" (r7_out),
84	87	: [number] "{$2}" (@backingInt(number)),
85	88	[arg1] "{$4}" (arg1),
86	89	[arg2] "{$5}" (arg2),
...	...	@@ -100,6 +103,8 @@ pub fn syscall5(
100	103	arg4: syscall_arg_t,
101	104	arg5: syscall_arg_t,
102	105	) u32 {
	106	// $7 is both an input and a clobber.
	107	var r7_out: syscall_arg_t = undefined;
103	108	return asm volatile (
104	109	\\ subu $sp, $sp, 24
105	110	\\ sw %[arg5], 16($sp)
...	...	@@ -109,6 +114,7 @@ pub fn syscall5(
109	114	\\ subu $v0, $zero, $v0
110	115	\\1:
111	116	: [ret] "={$2}" (-> u32),
	117	[r7_out] "={$7}" (r7_out),
112	118	: [number] "{$2}" (@backingInt(number)),
113	119	[arg1] "{$4}" (arg1),
114	120	[arg2] "{$5}" (arg2),
...	...	@@ -127,6 +133,8 @@ pub fn syscall6(
127	133	arg5: syscall_arg_t,
128	134	arg6: syscall_arg_t,
129	135	) u32 {
	136	// $7 is both an input and a clobber.
	137	var r7_out: syscall_arg_t = undefined;
130	138	return asm volatile (
131	139	\\ subu $sp, $sp, 24
132	140	\\ sw %[arg5], 16($sp)
...	...	@@ -137,6 +145,7 @@ pub fn syscall6(
137	145	\\ subu $v0, $zero, $v0
138	146	\\1:
139	147	: [ret] "={$2}" (-> u32),
	148	[r7_out] "={$7}" (r7_out),
140	149	: [number] "{$2}" (@backingInt(number)),
141	150	[arg1] "{$4}" (arg1),
142	151	[arg2] "{$5}" (arg2),
...	...	@@ -157,6 +166,8 @@ pub fn syscall7(
157	166	arg6: syscall_arg_t,
158	167	arg7: syscall_arg_t,
159	168	) u32 {
	169	// $7 is both an input and a clobber.
	170	var r7_out: syscall_arg_t = undefined;
160	171	return asm volatile (
161	172	\\ subu $sp, $sp, 32
162	173	\\ sw %[arg5], 16($sp)
...	...	@@ -168,6 +179,7 @@ pub fn syscall7(
168	179	\\ subu $v0, $zero, $v0
169	180	\\1:
170	181	: [ret] "={$2}" (-> u32),
	182	[r7_out] "={$7}" (r7_out),
171	183	: [number] "{$2}" (@backingInt(number)),
172	184	[arg1] "{$4}" (arg1),
173	185	[arg2] "{$5}" (arg2),

lib/std/os/linux/mips64.zig+9

...	...	@@ -75,12 +75,15 @@ pub fn syscall4(
75	75	arg3: syscall_arg_t,
76	76	arg4: syscall_arg_t,
77	77	) u64 {
	78	// $7 is both an input and a clobber.
	79	var r7_out: syscall_arg_t = undefined;
78	80	return asm volatile (
79	81	\\ syscall
80	82	\\ beq $a3, $zero, 1f
81	83	\\ dsubu $v0, $zero, $v0
82	84	\\1:
83	85	: [ret] "={$2}" (-> u64),
	86	[r7_out] "={$7}" (r7_out),
84	87	: [number] "{$2}" (@backingInt(number)),
85	88	[arg1] "{$4}" (arg1),
86	89	[arg2] "{$5}" (arg2),
...	...	@@ -97,12 +100,15 @@ pub fn syscall5(
97	100	arg4: syscall_arg_t,
98	101	arg5: syscall_arg_t,
99	102	) u64 {
	103	// $7 is both an input and a clobber.
	104	var r7_out: syscall_arg_t = undefined;
100	105	return asm volatile (
101	106	\\ syscall
102	107	\\ beq $a3, $zero, 1f
103	108	\\ dsubu $v0, $zero, $v0
104	109	\\1:
105	110	: [ret] "={$2}" (-> u64),
	111	[r7_out] "={$7}" (r7_out),
106	112	: [number] "{$2}" (@backingInt(number)),
107	113	[arg1] "{$4}" (arg1),
108	114	[arg2] "{$5}" (arg2),
...	...	@@ -121,12 +127,15 @@ pub fn syscall6(
121	127	arg5: syscall_arg_t,
122	128	arg6: syscall_arg_t,
123	129	) u64 {
	130	// $7 is both an input and a clobber.
	131	var r7_out: syscall_arg_t = undefined;
124	132	return asm volatile (
125	133	\\ syscall
126	134	\\ beq $a3, $zero, 1f
127	135	\\ dsubu $v0, $zero, $v0
128	136	\\1:
129	137	: [ret] "={$2}" (-> u64),
	138	[r7_out] "={$7}" (r7_out),
130	139	: [number] "{$2}" (@backingInt(number)),
131	140	[arg1] "{$4}" (arg1),
132	141	[arg2] "{$5}" (arg2),

lib/std/os/linux/mipsn32.zig+9

...	...	@@ -75,12 +75,15 @@ pub fn syscall4(
75	75	arg3: syscall_arg_t,
76	76	arg4: syscall_arg_t,
77	77	) u32 {
	78	// $7 is both an input and a clobber.
	79	var r7_out: syscall_arg_t = undefined;
78	80	return asm volatile (
79	81	\\ syscall
80	82	\\ beq $a3, $zero, 1f
81	83	\\ subu $v0, $zero, $v0
82	84	\\1:
83	85	: [ret] "={$2}" (-> u32),
	86	[r7_out] "={$7}" (r7_out),
84	87	: [number] "{$2}" (@backingInt(number)),
85	88	[arg1] "{$4}" (arg1),
86	89	[arg2] "{$5}" (arg2),
...	...	@@ -97,12 +100,15 @@ pub fn syscall5(
97	100	arg4: syscall_arg_t,
98	101	arg5: syscall_arg_t,
99	102	) u32 {
	103	// $7 is both an input and a clobber.
	104	var r7_out: syscall_arg_t = undefined;
100	105	return asm volatile (
101	106	\\ syscall
102	107	\\ beq $a3, $zero, 1f
103	108	\\ subu $v0, $zero, $v0
104	109	\\1:
105	110	: [ret] "={$2}" (-> u32),
	111	[r7_out] "={$7}" (r7_out),
106	112	: [number] "{$2}" (@backingInt(number)),
107	113	[arg1] "{$4}" (arg1),
108	114	[arg2] "{$5}" (arg2),
...	...	@@ -121,12 +127,15 @@ pub fn syscall6(
121	127	arg5: syscall_arg_t,
122	128	arg6: syscall_arg_t,
123	129	) u32 {
	130	// $7 is both an input and a clobber.
	131	var r7_out: syscall_arg_t = undefined;
124	132	return asm volatile (
125	133	\\ syscall
126	134	\\ beq $a3, $zero, 1f
127	135	\\ subu $v0, $zero, $v0
128	136	\\1:
129	137	: [ret] "={$2}" (-> u32),
	138	[r7_out] "={$7}" (r7_out),
130	139	: [number] "{$2}" (@backingInt(number)),
131	140	[arg1] "{$4}" (arg1),
132	141	[arg2] "{$5}" (arg2),