selftests/x86/thunks.S

0001 /* SPDX-License-Identifier: GPL-2.0-only */
0002 /*
0003  * thunks.S - assembly helpers for mixed-bitness code
0004  * Copyright (c) 2015 Andrew Lutomirski
0005  *
0006  * These are little helpers that make it easier to switch bitness on
0007  * the fly.
0008  */
0009
0010     .text
0011
0012     .global call32_from_64
0013     .type call32_from_64, @function
0014 call32_from_64:
0015     // rdi: stack to use
0016     // esi: function to call
0017
0018     // Save registers
0019     pushq %rbx
0020     pushq %rbp
0021     pushq %r12
0022     pushq %r13
0023     pushq %r14
0024     pushq %r15
0025     pushfq
0026
0027     // Switch stacks
0028     mov %rsp,(%rdi)
0029     mov %rdi,%rsp
0030
0031     // Switch to compatibility mode
0032     pushq $0x23  /* USER32_CS */
0033     pushq $1f
0034     lretq
0035
0036 1:
0037     .code32
0038     // Call the function
0039     call *%esi
0040     // Switch back to long mode
0041     jmp $0x33,$1f
0042     .code64
0043
0044 1:
0045     // Restore the stack
0046     mov (%rsp),%rsp
0047
0048     // Restore registers
0049     popfq
0050     popq %r15
0051     popq %r14
0052     popq %r13
0053     popq %r12
0054     popq %rbp
0055     popq %rbx
0056
0057     ret
0058
0059 .size call32_from_64, .-call32_from_64
0060
0061 .section .note.GNU-stack,"",%progbits