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    Subject[RFC/PATCH 1/7] x86/entry/32: Remove 32-bit syscall audit optimizations
    Date
    The asm audit optimizations are ugly and obfuscate the code too
    much. Remove them.

    This will regress performance if syscall auditing is enabled on
    32-bit kernels and sysenter is in use. If this becomes a problem,
    interested parties are encouraged to implement the equivalent of the
    64-bit opportunistic sysret optimization.

    Alternatively, a case could be made that, on 32-bit kernels, a less
    messy asm audit optimization could be done. 32-bit kernels don't have
    the complicated partial register saving tricks that 64-bit kernels
    have, so the sysenter post-syscall path could just call the audit
    hooks directly. Any reimplementation of this ought to demonstrate
    that it only calls the audit hook once per syscall, though, which does
    not currently appear to be true. Someone would have to make the case
    that doing so would be better than implementing opportunistic sysexit,
    though.

    Signed-off-by: Andy Lutomirski <luto@kernel.org>
    ---
    arch/x86/entry/entry_32.S | 48 ++---------------------------------------------
    1 file changed, 2 insertions(+), 46 deletions(-)

    diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
    index 21dc60a60b5f..90f9e7f6c15e 100644
    --- a/arch/x86/entry/entry_32.S
    +++ b/arch/x86/entry/entry_32.S
    @@ -45,16 +45,6 @@
    #include <asm/asm.h>
    #include <asm/smap.h>

    -/* Avoid __ASSEMBLER__'ifying <linux/audit.h> just for this. */
    -#include <linux/elf-em.h>
    -#define AUDIT_ARCH_I386 (EM_386|__AUDIT_ARCH_LE)
    -#define __AUDIT_ARCH_LE 0x40000000
    -
    -#ifndef CONFIG_AUDITSYSCALL
    -# define sysenter_audit syscall_trace_entry
    -# define sysexit_audit syscall_exit_work
    -#endif
    -
    .section .entry.text, "ax"

    /*
    @@ -339,7 +329,7 @@ sysenter_past_esp:
    GET_THREAD_INFO(%ebp)

    testl $_TIF_WORK_SYSCALL_ENTRY, TI_flags(%ebp)
    - jnz sysenter_audit
    + jnz syscall_trace_entry
    sysenter_do_call:
    cmpl $(NR_syscalls), %eax
    jae sysenter_badsys
    @@ -351,7 +341,7 @@ sysenter_after_call:
    TRACE_IRQS_OFF
    movl TI_flags(%ebp), %ecx
    testl $_TIF_ALLWORK_MASK, %ecx
    - jnz sysexit_audit
    + jnz syscall_exit_work
    sysenter_exit:
    /* if something modifies registers it must also disable sysexit */
    movl PT_EIP(%esp), %edx
    @@ -362,40 +352,6 @@ sysenter_exit:
    PTGS_TO_GS
    ENABLE_INTERRUPTS_SYSEXIT

    -#ifdef CONFIG_AUDITSYSCALL
    -sysenter_audit:
    - testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT), TI_flags(%ebp)
    - jnz syscall_trace_entry
    - /* movl PT_EAX(%esp), %eax already set, syscall number: 1st arg to audit */
    - movl PT_EBX(%esp), %edx /* ebx/a0: 2nd arg to audit */
    - /* movl PT_ECX(%esp), %ecx already set, a1: 3nd arg to audit */
    - pushl PT_ESI(%esp) /* a3: 5th arg */
    - pushl PT_EDX+4(%esp) /* a2: 4th arg */
    - call __audit_syscall_entry
    - popl %ecx /* get that remapped edx off the stack */
    - popl %ecx /* get that remapped esi off the stack */
    - movl PT_EAX(%esp), %eax /* reload syscall number */
    - jmp sysenter_do_call
    -
    -sysexit_audit:
    - testl $(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT), %ecx
    - jnz syscall_exit_work
    - TRACE_IRQS_ON
    - ENABLE_INTERRUPTS(CLBR_ANY)
    - movl %eax, %edx /* second arg, syscall return value */
    - cmpl $-MAX_ERRNO, %eax /* is it an error ? */
    - setbe %al /* 1 if so, 0 if not */
    - movzbl %al, %eax /* zero-extend that */
    - call __audit_syscall_exit
    - DISABLE_INTERRUPTS(CLBR_ANY)
    - TRACE_IRQS_OFF
    - movl TI_flags(%ebp), %ecx
    - testl $(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT), %ecx
    - jnz syscall_exit_work
    - movl PT_EAX(%esp), %eax /* reload syscall return value */
    - jmp sysenter_exit
    -#endif
    -
    .pushsection .fixup, "ax"
    2: movl $0, PT_FS(%esp)
    jmp 1b
    --
    2.4.3


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