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authorLinus Torvalds <torvalds@linux-foundation.org>2008-04-10 17:35:23 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2008-04-10 17:35:23 -0700
commitd10d89ec78114f925f63c5126a2b2490f501a462 (patch)
tree16b617325619c841ec1de6fe11b3f8f8f92fdd04
parent598af051a79d05b751fe793f1fe09fcf74763e02 (diff)
downloadlinux-d10d89ec78114f925f63c5126a2b2490f501a462.tar.bz2
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>
-rw-r--r--include/asm-x86/linkage.h11
-rw-r--r--include/linux/linkage.h13
2 files changed, 24 insertions, 0 deletions
diff --git a/include/asm-x86/linkage.h b/include/asm-x86/linkage.h
index d605eeba0f70..c048353f4b85 100644
--- a/include/asm-x86/linkage.h
+++ b/include/asm-x86/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...) \
diff --git a/include/linux/linkage.h b/include/linux/linkage.h
index fe2a39c489b6..b163c5c40dbc 100644
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -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