[PATCH 1/2] x86/asm/entry/32: Explain stub32_clone logic

[Denys Vlasenko]

The reason for copying of %r8 to %rcx is quite non-obvious.
Add a comment which explains why it is done.

Fix indentation and trailing whitespace while at it.

Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
---
CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Borislav Petkov <bp@xxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
CC: Oleg Nesterov <oleg@xxxxxxxxxx>
CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
CC: Alexei Starovoitov <ast@xxxxxxxxxxxx>
CC: Will Drewry <wad@xxxxxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: x86@xxxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
 arch/x86/ia32/ia32entry.S | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 2ca052e..8e72256 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -562,9 +562,17 @@ GLOBAL(\label)
 
 	ALIGN
 GLOBAL(stub32_clone)
-	leaq sys_clone(%rip),%rax
+	leaq	sys_clone(%rip), %rax
+	/*
+	 * 32-bit clone API is clone(..., int tls_val, int *child_tidptr).
+	 * 64-bit clone API is clone(..., int *child_tidptr, int tls_val).
+	 * Native 64-bit kernel's sys_clone() implements the latter.
+	 * We need to swap args here. But since tls_val is in fact ignored
+	 * by sys_clone(), we can get away with an assignment
+	 * (arg4 = arg5) instead of a full swap:
+	 */
 	mov	%r8, %rcx
-	jmp  ia32_ptregs_common	
+	jmp	ia32_ptregs_common
 
 	ALIGN
 ia32_ptregs_common:
-- 
1.8.1.4

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