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Message-ID: <CALCETrVK1aRDui4n=Sc7NbT-jA-6jobTtJ996UxwRKDYU-YgNw@mail.gmail.com> Date: Mon, 23 Feb 2015 16:37:26 -0800 From: Andy Lutomirski <luto@...capital.net> To: Denys Vlasenko <dvlasenk@...hat.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Oleg Nesterov <oleg@...hat.com>, Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, Frederic Weisbecker <fweisbec@...il.com>, X86 ML <x86@...nel.org>, Alexei Starovoitov <ast@...mgrid.com>, Will Drewry <wad@...omium.org>, Kees Cook <keescook@...omium.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 1/6] x86: add comments about various syscall instructions, no code changes On Mon, Feb 23, 2015 at 4:12 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote: > 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. Applied. > > Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com> > CC: Linus Torvalds <torvalds@...ux-foundation.org> > CC: Oleg Nesterov <oleg@...hat.com> > CC: Borislav Petkov <bp@...en8.de> > CC: "H. Peter Anvin" <hpa@...or.com> > CC: Andy Lutomirski <luto@...capital.net> > CC: Frederic Weisbecker <fweisbec@...il.com> > CC: X86 ML <x86@...nel.org> > CC: Alexei Starovoitov <ast@...mgrid.com> > CC: Will Drewry <wad@...omium.org> > CC: Kees Cook <keescook@...omium.org> > CC: linux-kernel@...r.kernel.org > --- > arch/x86/ia32/ia32entry.S | 133 ++++++++++++++++++++++++++++----------------- > arch/x86/kernel/entry_64.S | 32 ++++++----- > 2 files changed, 102 insertions(+), 63 deletions(-) > > diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S > index e99f8a5..b567056 100644 > --- a/arch/x86/ia32/ia32entry.S > +++ b/arch/x86/ia32/ia32entry.S > @@ -99,22 +99,25 @@ ENDPROC(native_irq_enable_sysexit) > /* > * 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: > - * %eax System call number. > - * %ebx Arg1 > - * %ecx Arg2 > - * %edx Arg3 > - * %esi Arg4 > - * %edi Arg5 > - * %ebp user stack > - * 0(%ebp) Arg6 > - * > - * Interrupts off. > - * > + * eax system call number > + * ebx arg1 > + * ecx arg2 > + * edx arg3 > + * esi arg4 > + * edi arg5 > + * ebp user stack > + * 0(%ebp) arg6 > + * > * 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. > - */ > + */ > ENTRY(ia32_sysenter_target) > CFI_STARTPROC32 simple > CFI_SIGNAL_FRAME > @@ -128,6 +131,7 @@ ENTRY(ia32_sysenter_target) > * disabled irqs, here we enable it straight after entry: > */ > ENABLE_INTERRUPTS(CLBR_NONE) > + /* Construct iret frame (ss,rsp,rflags,cs,rip) */ > movl %ebp,%ebp /* zero extension */ > pushq_cfi $__USER32_DS > /*CFI_REL_OFFSET ss,0*/ > @@ -140,14 +144,19 @@ ENTRY(ia32_sysenter_target) > pushq_cfi $__USER32_CS > /*CFI_REL_OFFSET cs,0*/ > movl %eax, %eax > + /* Store thread_info->sysenter_return in rip stack slot */ > pushq_cfi %r10 > CFI_REL_OFFSET rip,0 > + /* Store orig_ax */ > pushq_cfi %rax > + /* Construct the rest of "struct pt_regs" */ > cld > ALLOC_PT_GPREGS_ON_STACK > 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 > 1: movl (%rbp),%ebp > _ASM_EXTABLE(1b,ia32_badarg) > @@ -184,6 +193,7 @@ sysexit_from_sys_call: > movl RIP(%rsp),%edx /* User %eip */ > CFI_REGISTER rip,rdx > RESTORE_RSI_RDI > + /* pop everything except ss,rsp,rflags slots */ > REMOVE_PT_GPREGS_FROM_STACK 3*8 > xorq %r8,%r8 > xorq %r9,%r9 > @@ -194,6 +204,10 @@ sysexit_from_sys_call: > popq_cfi %rcx /* User %esp */ > CFI_REGISTER rsp,rcx > TRACE_IRQS_ON > + /* > + * 32bit SYSEXIT restores eip from edx, esp from ecx. > + * cs and ss are loaded from MSRs. > + */ > ENABLE_INTERRUPTS_SYSEXIT32 > > CFI_RESTORE_STATE > @@ -274,23 +288,33 @@ ENDPROC(ia32_sysenter_target) > /* > * 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: > - * %eax System call number. > - * %ebx Arg1 > - * %ecx return EIP > - * %edx Arg3 > - * %esi Arg4 > - * %edi Arg5 > - * %ebp Arg2 [note: not saved in the stack frame, should not be touched] > - * %esp user stack > - * 0(%esp) Arg6 > - * > - * Interrupts off. > - * > + * eax system call number > + * ecx return address > + * ebx arg1 > + * ebp arg2 (note: not saved in the stack frame, should not be touched) > + * edx arg3 > + * esi arg4 > + * edi arg5 > + * esp user stack > + * 0(%esp) arg6 > + * > * 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 > - * with the int 0x80 path. > - */ > + * path below. We set up a complete hardware stack frame to share code > + * with the int 0x80 path. > + */ > ENTRY(ia32_cstar_target) > CFI_STARTPROC32 simple > CFI_SIGNAL_FRAME > @@ -306,7 +330,7 @@ ENTRY(ia32_cstar_target) > * disabled irqs and here we enable it straight after entry: > */ > 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 > movl %eax,%eax /* zero extension */ > movq %rax,ORIG_RAX(%rsp) > @@ -320,9 +344,11 @@ ENTRY(ia32_cstar_target) > /*CFI_REL_OFFSET rflags,EFLAGS*/ > movq %r8,RSP(%rsp) > CFI_REL_OFFSET rsp,RSP > - /* no need to do an access_ok check here because r8 has been > - 32bit zero extended */ > - /* hardware stack frame is complete now */ > + /* iret stack frame is complete now */ > + /* > + * no need to do an access_ok check here because r8 has been > + * 32bit zero extended > + */ > ASM_STAC > 1: movl (%r8),%r9d > _ASM_EXTABLE(1b,ia32_badarg) > @@ -355,8 +381,15 @@ sysretl_from_sys_call: > TRACE_IRQS_ON > movl RSP(%rsp),%esp > 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 > - > + > #ifdef CONFIG_AUDITSYSCALL > cstar_auditsys: > CFI_RESTORE_STATE > @@ -394,26 +427,26 @@ ia32_badarg: > jmp ia32_sysret > CFI_ENDPROC > > -/* > - * Emulated IA32 system calls via int 0x80. > +/* > + * Emulated IA32 system calls via int 0x80. > * > - * Arguments: > - * %eax System call number. > - * %ebx Arg1 > - * %ecx Arg2 > - * %edx Arg3 > - * %esi Arg4 > - * %edi Arg5 > - * %ebp Arg6 [note: not saved in the stack frame, should not be touched] > + * Arguments: > + * eax system call number > + * ebx arg1 > + * ecx arg2 > + * edx arg3 > + * esi arg4 > + * edi arg5 > + * ebp arg6 (note: not saved in the stack frame, should not be touched) > * > * Notes: > - * Uses the same stack frame as the x86-64 version. > - * All registers except %eax must be saved (but ptrace may violate that) > + * Uses the same stack frame as the x86-64 version. > + * All registers except eax must be saved (but ptrace may violate that). > * 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 > * 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) > CFI_STARTPROC32 simple > @@ -432,7 +465,7 @@ ENTRY(ia32_syscall) > */ > ENABLE_INTERRUPTS(CLBR_NONE) > movl %eax,%eax > - pushq_cfi %rax > + pushq_cfi %rax /* store orig_ax */ > cld > /* note the registers are not zero extended to the sf. > this could be a problem. */ > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S > index be2b14c..63e7ccd 100644 > --- a/arch/x86/kernel/entry_64.S > +++ b/arch/x86/kernel/entry_64.S > @@ -256,25 +256,25 @@ ENTRY(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 > - * stack pointer. However, it does mask the flags register for us, so > - * CLD and CLAC are not needed. > - */ > - > -/* > - * Register setup: > + * 64bit 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. > + * > + * Registers on entry: > * rax system call number > + * rcx return address > + * r11 saved rflags (note: r11 is callee-clobbered register in C ABI) > * rdi arg0 > - * rcx return address for syscall/sysret, C arg3 > * rsi arg1 > * rdx arg2 > - * r10 arg3 (--> moved to rcx for C) > + * r10 arg3 (needs to be moved to rcx to conform to C ABI) > * r8 arg4 > * r9 arg5 > - * r11 eflags for syscall/sysret, temporary for C > - * r12-r15,rbp,rbx saved by C code, not touched. > + * (note: r12-r15,rbp,rbx are callee-preserved in C ABI) > * > * Interrupts are off on entry. > * Only called from user space. > @@ -302,13 +302,14 @@ ENTRY(system_call) > GLOBAL(system_call_after_swapgs) > > 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 > /* > * No need to follow this irqs off/on section - it's straight > * and short: > */ > 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 > movq $-ENOSYS,RAX(%rsp) > movq_cfi rax,ORIG_RAX > @@ -348,6 +349,11 @@ ret_from_sys_call: > CFI_REGISTER rip,rcx > /*CFI_REGISTER rflags,r11*/ > 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 > > CFI_RESTORE_STATE > -- > 1.8.1.4 > -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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