Add commentary about the new "asmlinkage_protect()" macro

It's really a pretty ugly thing to need, and some day it will hopefully be obviated by teaching gcc about the magic calling conventions for the low-level system call code, but in the meantime we can at least add big honking comments about why we need these insane and strange macros. I took my comments from my version of the macro, but I ended up deciding to just pick Roland's version of the actual code instead (with his prettier syntax that uses vararg macros). Thus the previous two commits that actually implement it. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Add commentary about the new "asmlinkage_protect()" macro
It's really a pretty ugly thing to need, and some day it will hopefully be obviated by teaching gcc about the magic calling conventions for the low-level system call code, but in the meantime we can at least add big honking comments about why we need these insane and strange macros. I took my comments from my version of the macro, but I ended up deciding to just pick Roland's version of the actual code instead (with his prettier syntax that uses vararg macros). Thus the previous two commits that actually implement it. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Linus Torvalds
1 parent 598af051a7
Showing 2 changed files with 24 additions and 0 deletions Side-by-side Diff
include/asm-x86/linkage.h
include/linux/linkage.h
@@ -14,6 +14,17 @@
  */
 #define asmregparm __attribute__((regparm(3)))
  
+/*
+ * Make sure the compiler doesn't do anything stupid with the
+ * arguments on the stack - they are owned by the *caller*, not
+ * the callee. This just fools gcc into not spilling into them,
+ * and keeps it from doing tailcall recursion and/or using the
+ * stack slots for temporaries, since they are live and "used"
+ * all the way to the end of the function.
+ *
+ * NOTE! On x86-64, all the arguments are in registers, so this
+ * only matters on a 32-bit kernel.
+ */
 #define asmlinkage_protect(n, ret, args...) \
 	__asmlinkage_protect##n(ret, ##args)
 #define __asmlinkage_protect_n(ret, args...) \
@@ -17,6 +17,19 @@
 # define asmregparm
 #endif
  
+/*
+ * This is used by architectures to keep arguments on the stack
+ * untouched by the compiler by keeping them live until the end.
+ * The argument stack may be owned by the assembly-language
+ * caller, not the callee, and gcc doesn't always understand
+ * that.
+ *
+ * We have the return value, and a maximum of six arguments.
+ *
+ * This should always be followed by a "return ret" for the
+ * protection to work (ie no more work that the compiler might
+ * end up needing stack temporaries for).
+ */
 #ifndef asmlinkage_protect
 # define asmlinkage_protect(n, ret, args...)	do { } while (0)
 #endif
...	...	@@ -14,6 +14,17 @@
14	14	*/
15	15	#define asmregparm __attribute__((regparm(3)))
16	16
	17	+/*
	18	+ * Make sure the compiler doesn't do anything stupid with the
	19	+ * arguments on the stack - they are owned by the caller, not
	20	+ * the callee. This just fools gcc into not spilling into them,
	21	+ * and keeps it from doing tailcall recursion and/or using the
	22	+ * stack slots for temporaries, since they are live and "used"
	23	+ * all the way to the end of the function.
	24	+ *
	25	+ * NOTE! On x86-64, all the arguments are in registers, so this
	26	+ * only matters on a 32-bit kernel.
	27	+ */
17	28	#define asmlinkage_protect(n, ret, args...) \
18	29	__asmlinkage_protect##n(ret, ##args)
19	30	#define __asmlinkage_protect_n(ret, args...) \
...	...	@@ -17,6 +17,19 @@
17	17	# define asmregparm
18	18	#endif
19	19
	20	+/*
	21	+ * This is used by architectures to keep arguments on the stack
	22	+ * untouched by the compiler by keeping them live until the end.
	23	+ * The argument stack may be owned by the assembly-language
	24	+ * caller, not the callee, and gcc doesn't always understand
	25	+ * that.
	26	+ *
	27	+ * We have the return value, and a maximum of six arguments.
	28	+ *
	29	+ * This should always be followed by a "return ret" for the
	30	+ * protection to work (ie no more work that the compiler might
	31	+ * end up needing stack temporaries for).
	32	+ */
20	33	#ifndef asmlinkage_protect
21	34	# define asmlinkage_protect(n, ret, args...) do { } while (0)
22	35	#endif