arch/x86/entry/vdso/vdso32/system_call.S

   1 /*
   2  * AT_SYSINFO entry point
   3 */
   4
   5 #include <linux/linkage.h>
   6 #include <asm/dwarf2.h>
   7 #include <asm/cpufeatures.h>
   8 #include <asm/alternative-asm.h>
   9
  10         .text
  11         .globl __kernel_vsyscall
  12         .type __kernel_vsyscall,@function
  13         ALIGN
  14 __kernel_vsyscall:
  15         CFI_STARTPROC
  16         /*
  17          * Reshuffle regs so that all of any of the entry instructions
  18          * will preserve enough state.
  19          *
  20          * A really nice entry sequence would be:
  21          *  pushl %edx
  22          *  pushl %ecx
  23          *  movl  %esp, %ecx
  24          *
  25          * Unfortunately, naughty Android versions between July and December
  26          * 2015 actually hardcode the traditional Linux SYSENTER entry
  27          * sequence.  That is severely broken for a number of reasons (ask
  28          * anyone with an AMD CPU, for example).  Nonetheless, we try to keep
  29          * it working approximately as well as it ever worked.
  30          *
  31          * This link may eludicate some of the history:
  32          *   https://android-review.googlesource.com/#/q/Iac3295376d61ef83e713ac9b528f3b50aa780cd7
  33          * personally, I find it hard to understand what's going on there.
  34          *
  35          * Note to future user developers: DO NOT USE SYSENTER IN YOUR CODE.
  36          * Execute an indirect call to the address in the AT_SYSINFO auxv
  37          * entry.  That is the ONLY correct way to make a fast 32-bit system
  38          * call on Linux.  (Open-coding int $0x80 is also fine, but it's
  39          * slow.)
  40          */
  41         pushl   %ecx
  42         CFI_ADJUST_CFA_OFFSET   4
  43         CFI_REL_OFFSET          ecx, 0
  44         pushl   %edx
  45         CFI_ADJUST_CFA_OFFSET   4
  46         CFI_REL_OFFSET          edx, 0
  47         pushl   %ebp
  48         CFI_ADJUST_CFA_OFFSET   4
  49         CFI_REL_OFFSET          ebp, 0
  50
  51         #define SYSENTER_SEQUENCE       "movl %esp, %ebp; sysenter"
  52         #define SYSCALL_SEQUENCE        "movl %ecx, %ebp; syscall"
  53
  54 #ifdef CONFIG_X86_64
  55         /* If SYSENTER (Intel) or SYSCALL32 (AMD) is available, use it. */
  56         ALTERNATIVE_2 "", SYSENTER_SEQUENCE, X86_FEATURE_SYSENTER32, \
  57                           SYSCALL_SEQUENCE,  X86_FEATURE_SYSCALL32
  58 #else
  59         ALTERNATIVE "", SYSENTER_SEQUENCE, X86_FEATURE_SEP
  60 #endif
  61
  62         /* Enter using int $0x80 */
  63         int     $0x80
  64 GLOBAL(int80_landing_pad)
  65
  66         /*
  67          * Restore EDX and ECX in case they were clobbered.  EBP is not
  68          * clobbered (the kernel restores it), but it's cleaner and
  69          * probably faster to pop it than to adjust ESP using addl.
  70          */
  71         popl    %ebp
  72         CFI_RESTORE             ebp
  73         CFI_ADJUST_CFA_OFFSET   -4
  74         popl    %edx
  75         CFI_RESTORE             edx
  76         CFI_ADJUST_CFA_OFFSET   -4
  77         popl    %ecx
  78         CFI_RESTORE             ecx
  79         CFI_ADJUST_CFA_OFFSET   -4
  80         ret
  81         CFI_ENDPROC
  82
  83         .size __kernel_vsyscall,.-__kernel_vsyscall
  84         .previous