[PATCH v2 12/12] x86/entry: Improve system call entry comments

From: Andy Lutomirski
Date: Wed Mar 09 2016 - 22:01:46 EST


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@xxxxxxxxxx>
---
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 13a7f41c1886..587f3eb8c524 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 70eadb0ea5fa..858b555e274b 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 26469c11796d..847f2f0c31e5 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.
--
2.5.0