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x86/asm/entry: Add comments about various syscall instructions

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SYSCALL/SYSRET and SYSENTER/SYSEXIT have weird semantics.
Moreover, they differ in 32- and 64-bit mode.

What is saved? What is not? Is rsp set? Are interrupts disabled?
People tend to not remember these details well enough.

This patch adds comments which explain in detail
what registers are modified by each of these instructions.

The comments are placed immediately before corresponding
entry and exit points.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Borislav Petkov <bp@alien8.de>
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: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/a94b98b63527797c871a81402ff5060b18fa880a.1424989793.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Denys Vlasenko 2015-02-26 14:40:32 -08:00 committed by Ingo Molnar
parent 050273d19b
commit b87cf63e2a
2 changed files with 102 additions and 63 deletions
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133
arch/x86/ia32/ia32entry.S
View File

@ -99,22 +99,25 @@ ENDPROC(native_irq_enable_sysexit)
/* /*
* 32bit SYSENTER instruction entry. * 32bit SYSENTER instruction entry.
* *
* SYSENTER loads ss, rsp, cs, and rip from previously programmed MSRs.
* IF and VM in rflags are cleared (IOW: interrupts are off).
* SYSENTER does not save anything on the stack,
* and does not save old rip (!!!) and rflags.
*
* Arguments: * Arguments:
* %eax System call number. * eax system call number
* %ebx Arg1 * ebx arg1
* %ecx Arg2 * ecx arg2
* %edx Arg3 * edx arg3
* %esi Arg4 * esi arg4
* %edi Arg5 * edi arg5
* %ebp user stack * ebp user stack
* 0(%ebp) Arg6 * 0(%ebp) arg6
* *
* Interrupts off.
*
* This is purely a fast path. For anything complicated we use the int 0x80 * This is purely a fast path. For anything complicated we use the int 0x80
* path below. Set up a complete hardware stack frame to share code * path below. We set up a complete hardware stack frame to share code
* with the int 0x80 path. * with the int 0x80 path.
*/ */
ENTRY(ia32_sysenter_target) ENTRY(ia32_sysenter_target)
CFI_STARTPROC32 simple CFI_STARTPROC32 simple
CFI_SIGNAL_FRAME CFI_SIGNAL_FRAME
@ -128,6 +131,7 @@ ENTRY(ia32_sysenter_target)
* disabled irqs, here we enable it straight after entry: * disabled irqs, here we enable it straight after entry:
*/ */
ENABLE_INTERRUPTS(CLBR_NONE) ENABLE_INTERRUPTS(CLBR_NONE)
/* Construct iret frame (ss,rsp,rflags,cs,rip) */
movl %ebp,%ebp /* zero extension */ movl %ebp,%ebp /* zero extension */
pushq_cfi $__USER32_DS pushq_cfi $__USER32_DS
/*CFI_REL_OFFSET ss,0*/ /*CFI_REL_OFFSET ss,0*/
@ -140,14 +144,19 @@ ENTRY(ia32_sysenter_target)
pushq_cfi $__USER32_CS pushq_cfi $__USER32_CS
/*CFI_REL_OFFSET cs,0*/ /*CFI_REL_OFFSET cs,0*/
movl %eax, %eax movl %eax, %eax
/* Store thread_info->sysenter_return in rip stack slot */
pushq_cfi %r10 pushq_cfi %r10
CFI_REL_OFFSET rip,0 CFI_REL_OFFSET rip,0
/* Store orig_ax */
pushq_cfi %rax pushq_cfi %rax
/* Construct the rest of "struct pt_regs" */
cld cld
ALLOC_PT_GPREGS_ON_STACK ALLOC_PT_GPREGS_ON_STACK
SAVE_C_REGS_EXCEPT_R891011 SAVE_C_REGS_EXCEPT_R891011
/* no need to do an access_ok check here because rbp has been /*
32bit zero extended */ * no need to do an access_ok check here because rbp has been
* 32bit zero extended
*/
ASM_STAC ASM_STAC
1: movl (%rbp),%ebp 1: movl (%rbp),%ebp
_ASM_EXTABLE(1b,ia32_badarg) _ASM_EXTABLE(1b,ia32_badarg)
@ -184,6 +193,7 @@ sysexit_from_sys_call:
movl RIP(%rsp),%edx /* User %eip */ movl RIP(%rsp),%edx /* User %eip */
CFI_REGISTER rip,rdx CFI_REGISTER rip,rdx
RESTORE_RSI_RDI RESTORE_RSI_RDI
/* pop everything except ss,rsp,rflags slots */
REMOVE_PT_GPREGS_FROM_STACK 3*8 REMOVE_PT_GPREGS_FROM_STACK 3*8
xorq %r8,%r8 xorq %r8,%r8
xorq %r9,%r9 xorq %r9,%r9
@ -194,6 +204,10 @@ sysexit_from_sys_call:
popq_cfi %rcx /* User %esp */ popq_cfi %rcx /* User %esp */
CFI_REGISTER rsp,rcx CFI_REGISTER rsp,rcx
TRACE_IRQS_ON TRACE_IRQS_ON
/*
* 32bit SYSEXIT restores eip from edx, esp from ecx.
* cs and ss are loaded from MSRs.
*/
ENABLE_INTERRUPTS_SYSEXIT32 ENABLE_INTERRUPTS_SYSEXIT32
CFI_RESTORE_STATE CFI_RESTORE_STATE
@ -274,23 +288,33 @@ ENDPROC(ia32_sysenter_target)
/* /*
* 32bit SYSCALL instruction entry. * 32bit SYSCALL instruction entry.
* *
* 32bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
* then loads new ss, cs, and rip from previously programmed MSRs.
* rflags gets masked by a value from another MSR (so CLD and CLAC
* are not needed). SYSCALL does not save anything on the stack
* and does not change rsp.
*
* Note: rflags saving+masking-with-MSR happens only in Long mode
* (in legacy 32bit mode, IF, RF and VM bits are cleared and that's it).
* Don't get confused: rflags saving+masking depends on Long Mode Active bit
* (EFER.LMA=1), NOT on bitness of userspace where SYSCALL executes
* or target CS descriptor's L bit (SYSCALL does not read segment descriptors).
*
* Arguments: * Arguments:
* %eax System call number. * eax system call number
* %ebx Arg1 * ecx return address
* %ecx return EIP * ebx arg1
* %edx Arg3 * ebp arg2 (note: not saved in the stack frame, should not be touched)
* %esi Arg4 * edx arg3
* %edi Arg5 * esi arg4
* %ebp Arg2 [note: not saved in the stack frame, should not be touched] * edi arg5
* %esp user stack * esp user stack
* 0(%esp) Arg6 * 0(%esp) arg6
* *
* Interrupts off.
*
* This is purely a fast path. For anything complicated we use the int 0x80 * This is purely a fast path. For anything complicated we use the int 0x80
* path below. Set up a complete hardware stack frame to share code * path below. We set up a complete hardware stack frame to share code
* with the int 0x80 path. * with the int 0x80 path.
*/ */
ENTRY(ia32_cstar_target) ENTRY(ia32_cstar_target)
CFI_STARTPROC32 simple CFI_STARTPROC32 simple
CFI_SIGNAL_FRAME CFI_SIGNAL_FRAME
@ -306,7 +330,7 @@ ENTRY(ia32_cstar_target)
* disabled irqs and here we enable it straight after entry: * disabled irqs and here we enable it straight after entry:
*/ */
ENABLE_INTERRUPTS(CLBR_NONE) ENABLE_INTERRUPTS(CLBR_NONE)
ALLOC_PT_GPREGS_ON_STACK 8 ALLOC_PT_GPREGS_ON_STACK 8 /* +8: space for orig_ax */
SAVE_C_REGS_EXCEPT_RCX_R891011 SAVE_C_REGS_EXCEPT_RCX_R891011
movl %eax,%eax /* zero extension */ movl %eax,%eax /* zero extension */
movq %rax,ORIG_RAX(%rsp) movq %rax,ORIG_RAX(%rsp)
@ -320,9 +344,11 @@ ENTRY(ia32_cstar_target)
/*CFI_REL_OFFSET rflags,EFLAGS*/ /*CFI_REL_OFFSET rflags,EFLAGS*/
movq %r8,RSP(%rsp) movq %r8,RSP(%rsp)
CFI_REL_OFFSET rsp,RSP CFI_REL_OFFSET rsp,RSP
/* no need to do an access_ok check here because r8 has been /* iret stack frame is complete now */
32bit zero extended */ /*
/* hardware stack frame is complete now */ * no need to do an access_ok check here because r8 has been
* 32bit zero extended
*/
ASM_STAC ASM_STAC
1: movl (%r8),%r9d 1: movl (%r8),%r9d
_ASM_EXTABLE(1b,ia32_badarg) _ASM_EXTABLE(1b,ia32_badarg)
@ -355,8 +381,15 @@ sysretl_from_sys_call:
TRACE_IRQS_ON TRACE_IRQS_ON
movl RSP(%rsp),%esp movl RSP(%rsp),%esp
CFI_RESTORE rsp CFI_RESTORE rsp
/*
* 64bit->32bit SYSRET restores eip from ecx,
* eflags from r11 (but RF and VM bits are forced to 0),
* cs and ss are loaded from MSRs.
* (Note: 32bit->32bit SYSRET is different: since r11
* does not exist, it merely sets eflags.IF=1).
*/
USERGS_SYSRET32 USERGS_SYSRET32
#ifdef CONFIG_AUDITSYSCALL #ifdef CONFIG_AUDITSYSCALL
cstar_auditsys: cstar_auditsys:
CFI_RESTORE_STATE CFI_RESTORE_STATE
@ -394,26 +427,26 @@ ia32_badarg:
jmp ia32_sysret jmp ia32_sysret
CFI_ENDPROC CFI_ENDPROC
/* /*
* Emulated IA32 system calls via int 0x80. * Emulated IA32 system calls via int 0x80.
* *
* Arguments: * Arguments:
* %eax System call number. * eax system call number
* %ebx Arg1 * ebx arg1
* %ecx Arg2 * ecx arg2
* %edx Arg3 * edx arg3
* %esi Arg4 * esi arg4
* %edi Arg5 * edi arg5
* %ebp Arg6 [note: not saved in the stack frame, should not be touched] * ebp arg6 (note: not saved in the stack frame, should not be touched)
* *
* Notes: * Notes:
* Uses the same stack frame as the x86-64 version. * Uses the same stack frame as the x86-64 version.
* All registers except %eax must be saved (but ptrace may violate that) * All registers except eax must be saved (but ptrace may violate that).
* Arguments are zero extended. For system calls that want sign extension and * Arguments are zero extended. For system calls that want sign extension and
* take long arguments a wrapper is needed. Most calls can just be called * take long arguments a wrapper is needed. Most calls can just be called
* directly. * directly.
* Assumes it is only called from user space and entered with interrupts off. * Assumes it is only called from user space and entered with interrupts off.
*/ */
ENTRY(ia32_syscall) ENTRY(ia32_syscall)
CFI_STARTPROC32 simple CFI_STARTPROC32 simple
@ -432,7 +465,7 @@ ENTRY(ia32_syscall)
*/ */
ENABLE_INTERRUPTS(CLBR_NONE) ENABLE_INTERRUPTS(CLBR_NONE)
movl %eax,%eax movl %eax,%eax
pushq_cfi %rax pushq_cfi %rax /* store orig_ax */
cld cld
/* note the registers are not zero extended to the sf. /* note the registers are not zero extended to the sf.
this could be a problem. */ this could be a problem. */

32
arch/x86/kernel/entry_64.S
View File

@ -256,25 +256,25 @@ ENTRY(ret_from_fork)
END(ret_from_fork) END(ret_from_fork)
/* /*
* System call entry. Up to 6 arguments in registers are supported. * 64bit SYSCALL instruction entry. Up to 6 arguments in registers.
* *
* SYSCALL does not save anything on the stack and does not change the * 64bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
* stack pointer. However, it does mask the flags register for us, so * then loads new ss, cs, and rip from previously programmed MSRs.
* CLD and CLAC are not needed. * rflags gets masked by a value from another MSR (so CLD and CLAC
*/ * are not needed). SYSCALL does not save anything on the stack
* and does not change rsp.
/* *
* Register setup: * Registers on entry:
* rax system call number * rax system call number
* rcx return address
* r11 saved rflags (note: r11 is callee-clobbered register in C ABI)
* rdi arg0 * rdi arg0
* rcx return address for syscall/sysret, C arg3
* rsi arg1 * rsi arg1
* rdx arg2 * rdx arg2
* r10 arg3 (--> moved to rcx for C) * r10 arg3 (needs to be moved to rcx to conform to C ABI)
* r8 arg4 * r8 arg4
* r9 arg5 * r9 arg5
* r11 eflags for syscall/sysret, temporary for C * (note: r12-r15,rbp,rbx are callee-preserved in C ABI)
* r12-r15,rbp,rbx saved by C code, not touched.
* *
* Interrupts are off on entry. * Interrupts are off on entry.
* Only called from user space. * Only called from user space.
@ -302,13 +302,14 @@ ENTRY(system_call)
GLOBAL(system_call_after_swapgs) GLOBAL(system_call_after_swapgs)
movq %rsp,PER_CPU_VAR(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 movq PER_CPU_VAR(kernel_stack),%rsp
/* /*
* No need to follow this irqs off/on section - it's straight * No need to follow this irqs off/on section - it's straight
* and short: * and short:
*/ */
ENABLE_INTERRUPTS(CLBR_NONE) ENABLE_INTERRUPTS(CLBR_NONE)
ALLOC_PT_GPREGS_ON_STACK 8 ALLOC_PT_GPREGS_ON_STACK 8 /* +8: space for orig_ax */
SAVE_C_REGS_EXCEPT_RAX_RCX SAVE_C_REGS_EXCEPT_RAX_RCX
movq $-ENOSYS,RAX(%rsp) movq $-ENOSYS,RAX(%rsp)
movq_cfi rax,ORIG_RAX movq_cfi rax,ORIG_RAX
@ -348,6 +349,11 @@ ret_from_sys_call:
CFI_REGISTER rip,rcx CFI_REGISTER rip,rcx
/*CFI_REGISTER rflags,r11*/ /*CFI_REGISTER rflags,r11*/
movq PER_CPU_VAR(old_rsp), %rsp movq PER_CPU_VAR(old_rsp), %rsp
/*
* 64bit SYSRET restores rip from rcx,
* rflags from r11 (but RF and VM bits are forced to 0),
* cs and ss are loaded from MSRs.
*/
USERGS_SYSRET64 USERGS_SYSRET64
CFI_RESTORE_STATE CFI_RESTORE_STATE