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Commit 7fcb3bc3 authored by Ingo Molnar's avatar Ingo Molnar
Browse files

x86/asm/entry/64: Update comments about stack frames 



Tweak a few outdated comments that were obsoleted by recent changes
to syscall entry code:

 - we no longer have a "partial stack frame" on
   entry, ever.

 - explain the syscall entry usage of old_rsp.

Partially based on a (split out of) patch from Denys Vlasenko.

Originally-from: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: default avatarBorislav Petkov <bp@alien8.de>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent ac9af498

arch/x86/kernel/entry_64.S

+11 −8
Original line number Diff line number Diff line
@@ -15,10 +15,8 @@ 
 * after an interrupt and after each system call.

 *

 * A note on terminology:

 * - top of stack: Architecture defined interrupt frame from SS to RIP

 * - iret frame: Architecture defined interrupt frame from SS to RIP

 * at the top of the kernel process stack.

 * - partial stack frame: partially saved registers up to R11.

 * - full stack frame: Like partial stack frame, but all register saved.

 *

 * Some macro usage:

 * - CFI macros are used to generate dwarf2 unwind information for better
@@ -219,7 +217,7 @@ ENDPROC(native_usergs_sysret64) 
 * Interrupts are off on entry.

 * Only called from user space.

 *

 * When user can change the frames always force IRET. That is because

 * When user can change pt_regs->foo always force IRET. That is because

 * it deals with uncanonical addresses better. SYSRET has trouble

 * with them due to bugs in both AMD and Intel CPUs.

 */
@@ -238,6 +236,11 @@ ENTRY(system_call) 
	 */

GLOBAL(system_call_after_swapgs)



	/*

	 * We use 'old_rsp' as a scratch register, hence this block must execute

	 * atomically in the face of possible interrupt-driven task preemption,

	 * so we can enable interrupts only after we're done with using old_rsp:

	 */

	movq	%rsp,PER_CPU_VAR(old_rsp)

	/* kernel_stack is set so that 5 slots (iret frame) are preallocated */

	movq	PER_CPU_VAR(kernel_stack),%rsp
@@ -303,7 +306,7 @@ int_ret_from_sys_call_fixup: 
	FIXUP_TOP_OF_STACK %r11

	jmp int_ret_from_sys_call



	/* Do syscall tracing */

	/* Do syscall entry tracing */

tracesys:

	movq %rsp, %rdi

	movq $AUDIT_ARCH_X86_64, %rsi
@@ -339,11 +342,11 @@ tracesys_phase2: 
	movq %r10,%rcx	/* fixup for C */

	call *sys_call_table(,%rax,8)

	movq %rax,RAX(%rsp)

	/* Use IRET because user could have changed frame */

	/* Use IRET because user could have changed pt_regs->foo */



/*

 * Syscall return path ending with IRET.

 * Has correct top of stack, but partial stack frame.

 * Has correct iret frame.

 */

GLOBAL(int_ret_from_sys_call)

	DISABLE_INTERRUPTS(CLBR_NONE)
@@ -374,7 +377,7 @@ int_careful: 
	TRACE_IRQS_OFF

	jmp int_with_check



	/* handle signals and tracing -- both require a full stack frame */

	/* handle signals and tracing -- both require a full pt_regs */

int_very_careful:

	TRACE_IRQS_ON

	ENABLE_INTERRUPTS(CLBR_NONE)