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Message-ID: <tip-fda57b2267e12de014069b1596a5438cf76fc7c6@git.kernel.org> Date: Thu, 10 Mar 2016 03:05:03 -0800 From: tip-bot for Andy Lutomirski <tipbot@...or.com> To: linux-tip-commits@...r.kernel.org Cc: dvlasenk@...hat.com, hpa@...or.com, oleg@...hat.com, luto@...capital.net, luto@...nel.org, linux-kernel@...r.kernel.org, andrew.cooper3@...rix.com, bp@...en8.de, tglx@...utronix.de, peterz@...radead.org, torvalds@...ux-foundation.org, mingo@...nel.org, brgerst@...il.com Subject: [tip:x86/asm] x86/entry: Improve system call entry comments Commit-ID: fda57b2267e12de014069b1596a5438cf76fc7c6 Gitweb: http://git.kernel.org/tip/fda57b2267e12de014069b1596a5438cf76fc7c6 Author: Andy Lutomirski <luto@...nel.org> AuthorDate: Wed, 9 Mar 2016 19:00:35 -0800 Committer: Ingo Molnar <mingo@...nel.org> CommitDate: Thu, 10 Mar 2016 09:48:15 +0100 x86/entry: Improve system call entry comments 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@...nel.org> Cc: Andrew Cooper <andrew.cooper3@...rix.com> Cc: Andy Lutomirski <luto@...capital.net> Cc: Borislav Petkov <bp@...en8.de> Cc: Brian Gerst <brgerst@...il.com> Cc: Denys Vlasenko <dvlasenk@...hat.com> Cc: H. Peter Anvin <hpa@...or.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Oleg Nesterov <oleg@...hat.com> Cc: Peter Zijlstra <peterz@...radead.org> Cc: Thomas Gleixner <tglx@...utronix.de> Link: http://lkml.kernel.org/r/9524ecef7a295347294300045d08354d6a57c6e7.1457578375.git.luto@kernel.org Signed-off-by: Ingo Molnar <mingo@...nel.org> --- arch/x86/entry/entry_32.S | 61 +++++++++++++++++++++++++++- arch/x86/entry/entry_64.S | 10 +++++ arch/x86/entry/entry_64_compat.S | 85 +++++++++++++++++++++++++++------------- 3 files changed, 128 insertions(+), 28 deletions(-) diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S index 7b3ec24..286efa3 100644 --- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -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 */ diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 70eadb0..858b555 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -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 diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index cbb5e69..efe0fe3 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -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 system calls through the vDSO's __kernel_vsyscall enter here + * on 64-bit kernels running on AMD CPUs. + * + * 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. * - * 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. + * - 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. * - * Note: rflags saving+masking-with-MSR happens only in Long mode + * 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.
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