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x86/entry: Improve system call entry comments

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Ingo suggested that the comments should explain when the various
entries are used.  This adds these explanations and improves other
parts of the comments.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/9524ecef7a295347294300045d08354d6a57c6e7.1457578375.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2016-03-09 19:00:35 -08:00 committed by Ingo Molnar
parent 392a62549f
commit fda57b2267
3 changed files with 128 additions and 28 deletions
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61
arch/x86/entry/entry_32.S
View File

@ -307,6 +307,38 @@ ENTRY(xen_sysenter_target)
jmp sysenter_past_esp
#endif
/*
* 32-bit SYSENTER entry.
*
* 32-bit system calls through the vDSO's __kernel_vsyscall enter here
* if X86_FEATURE_SEP is available. This is the preferred system call
* entry on 32-bit systems.
*
* The SYSENTER instruction, in principle, should *only* occur in the
* vDSO. In practice, a small number of Android devices were shipped
* with a copy of Bionic that inlined a SYSENTER instruction. This
* never happened in any of Google's Bionic versions -- it only happened
* in a narrow range of Intel-provided versions.
*
* SYSENTER loads SS, ESP, CS, and EIP 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 EIP (!!!), ESP, or EFLAGS.
*
* To avoid losing track of EFLAGS.VM (and thus potentially corrupting
* user and/or vm86 state), we explicitly disable the SYSENTER
* instruction in vm86 mode by reprogramming the MSRs.
*
* Arguments:
* eax system call number
* ebx arg1
* ecx arg2
* edx arg3
* esi arg4
* edi arg5
* ebp user stack
* 0(%ebp) arg6
*/
ENTRY(entry_SYSENTER_32)
movl TSS_sysenter_sp0(%esp), %esp
sysenter_past_esp:
@ -397,7 +429,34 @@ sysenter_past_esp:
GLOBAL(__end_SYSENTER_singlestep_region)
ENDPROC(entry_SYSENTER_32)
# system call handler stub
/*
* 32-bit legacy system call entry.
*
* 32-bit x86 Linux system calls traditionally used the INT $0x80
* instruction. INT $0x80 lands here.
*
* This entry point can be used by any 32-bit perform system calls.
* Instances of INT $0x80 can be found inline in various programs and
* libraries. It is also used by the vDSO's __kernel_vsyscall
* fallback for hardware that doesn't support a faster entry method.
* Restarted 32-bit system calls also fall back to INT $0x80
* regardless of what instruction was originally used to do the system
* call. (64-bit programs can use INT $0x80 as well, but they can
* only run on 64-bit kernels and therefore land in
* entry_INT80_compat.)
*
* This is considered a slow path. It is not used by most libc
* implementations on modern hardware except during process startup.
*
* Arguments:
* eax system call number
* ebx arg1
* ecx arg2
* edx arg3
* esi arg4
* edi arg5
* ebp arg6
*/
ENTRY(entry_INT80_32)
ASM_CLAC
pushl %eax /* pt_regs->orig_ax */

10
arch/x86/entry/entry_64.S
View File

@ -103,6 +103,16 @@ ENDPROC(native_usergs_sysret64)
/*
* 64-bit SYSCALL instruction entry. Up to 6 arguments in registers.
*
* This is the only entry point used for 64-bit system calls. The
* hardware interface is reasonably well designed and the register to
* argument mapping Linux uses fits well with the registers that are
* available when SYSCALL is used.
*
* SYSCALL instructions can be found inlined in libc implementations as
* well as some other programs and libraries. There are also a handful
* of SYSCALL instructions in the vDSO used, for example, as a
* clock_gettimeofday fallback.
*
* 64-bit 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

85
arch/x86/entry/entry_64_compat.S
View File

@ -19,12 +19,21 @@
.section .entry.text, "ax"
/*
* 32-bit SYSENTER instruction entry.
* 32-bit SYSENTER entry.
*
* SYSENTER loads ss, rsp, cs, and rip from previously programmed MSRs.
* IF and VM in rflags are cleared (IOW: interrupts are off).
* 32-bit system calls through the vDSO's __kernel_vsyscall enter here
* on 64-bit kernels running on Intel CPUs.
*
* The SYSENTER instruction, in principle, should *only* occur in the
* vDSO. In practice, a small number of Android devices were shipped
* with a copy of Bionic that inlined a SYSENTER instruction. This
* never happened in any of Google's Bionic versions -- it only happened
* in a narrow range of Intel-provided versions.
*
* 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.
* and does not save old RIP (!!!), RSP, or RFLAGS.
*
* Arguments:
* eax system call number
@ -35,10 +44,6 @@
* edi arg5
* ebp user stack
* 0(%ebp) arg6
*
* This is purely a fast path. For anything complicated we use the int 0x80
* path below. We set up a complete hardware stack frame to share code
* with the int 0x80 path.
*/
ENTRY(entry_SYSENTER_compat)
/* Interrupts are off on entry. */
@ -131,17 +136,38 @@ GLOBAL(__end_entry_SYSENTER_compat)
ENDPROC(entry_SYSENTER_compat)
/*
* 32-bit SYSCALL instruction entry.
* 32-bit SYSCALL entry.
*
* 32-bit 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.
* 32-bit system calls through the vDSO's __kernel_vsyscall enter here
* on 64-bit kernels running on AMD CPUs.
*
* Note: rflags saving+masking-with-MSR happens only in Long mode
* The SYSCALL instruction, in principle, should *only* occur in the
* vDSO. In practice, it appears that this really is the case.
* As evidence:
*
* - The calling convention for SYSCALL has changed several times without
* anyone noticing.
*
* - Prior to the in-kernel X86_BUG_SYSRET_SS_ATTRS fixup, anything
* user task that did SYSCALL without immediately reloading SS
* would randomly crash.
*
* - Most programmers do not directly target AMD CPUs, and the 32-bit
* SYSCALL instruction does not exist on Intel CPUs. Even on AMD
* CPUs, Linux disables the SYSCALL instruction on 32-bit kernels
* because the SYSCALL instruction in legacy/native 32-bit mode (as
* opposed to compat mode) is sufficiently poorly designed as to be
* essentially unusable.
*
* 32-bit 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 32-bit 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
* 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).
*
@ -241,7 +267,21 @@ sysret32_from_system_call:
END(entry_SYSCALL_compat)
/*
* Emulated IA32 system calls via int 0x80.
* 32-bit legacy system call entry.
*
* 32-bit x86 Linux system calls traditionally used the INT $0x80
* instruction. INT $0x80 lands here.
*
* This entry point can be used by 32-bit and 64-bit programs to perform
* 32-bit system calls. Instances of INT $0x80 can be found inline in
* various programs and libraries. It is also used by the vDSO's
* __kernel_vsyscall fallback for hardware that doesn't support a faster
* entry method. Restarted 32-bit system calls also fall back to INT
* $0x80 regardless of what instruction was originally used to do the
* system call.
*
* This is considered a slow path. It is not used by most libc
* implementations on modern hardware except during process startup.
*
* Arguments:
* eax system call number
@ -250,17 +290,8 @@ END(entry_SYSCALL_compat)
* 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).
* 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.
* ebp arg6
*/
ENTRY(entry_INT80_compat)
/*
* Interrupts are off on entry.