• Andy Lutomirski's avatar
    x86/asm/irq: Stop relying on magic JMP behavior for early_idt_handlers · dd727f01
    Andy Lutomirski authored
    commit 425be567 upstream.
    
    The early_idt_handlers asm code generates an array of entry
    points spaced nine bytes apart.  It's not really clear from that
    code or from the places that reference it what's going on, and
    the code only works in the first place because GAS never
    generates two-byte JMP instructions when jumping to global
    labels.
    
    Clean up the code to generate the correct array stride (member size)
    explicitly. This should be considerably more robust against
    screw-ups, as GAS will warn if a .fill directive has a negative
    count.  Using '. =' to advance would have been even more robust
    (it would generate an actual error if it tried to move
    backwards), but it would pad with nulls, confusing anyone who
    tries to disassemble the code.  The new scheme should be much
    clearer to future readers.
    
    While we're at it, improve the comments and rename the array and
    common code.
    
    Binutils may start relaxing jumps to non-weak labels.  If so,
    this change will fix our build, and we may need to backport this
    change.
    
    Before, on x86_64:
    
      0000000000000000 <early_idt_handlers>:
         0:   6a 00                   pushq  $0x0
         2:   6a 00                   pushq  $0x0
         4:   e9 00 00 00 00          jmpq   9 <early_idt_handlers+0x9>
                              5: R_X86_64_PC32        early_idt_handler-0x4
      ...
        48:   66 90                   xchg   %ax,%ax
        4a:   6a 08                   pushq  $0x8
        4c:   e9 00 00 00 00          jmpq   51 <early_idt_handlers+0x51>
                              4d: R_X86_64_PC32       early_idt_handler-0x4
      ...
       117:   6a 00                   pushq  $0x0
       119:   6a 1f                   pushq  $0x1f
       11b:   e9 00 00 00 00          jmpq   120 <early_idt_handler>
                              11c: R_X86_64_PC32      early_idt_handler-0x4
    
    After:
    
      0000000000000000 <early_idt_handler_array>:
         0:   6a 00                   pushq  $0x0
         2:   6a 00                   pushq  $0x0
         4:   e9 14 01 00 00          jmpq   11d <early_idt_handler_common>
      ...
        48:   6a 08                   pushq  $0x8
        4a:   e9 d1 00 00 00          jmpq   120 <early_idt_handler_common>
        4f:   cc                      int3
        50:   cc                      int3
      ...
       117:   6a 00                   pushq  $0x0
       119:   6a 1f                   pushq  $0x1f
       11b:   eb 03                   jmp    120 <early_idt_handler_common>
       11d:   cc                      int3
       11e:   cc                      int3
       11f:   cc                      int3
    Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
    Acked-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
    Cc: Binutils <binutils@sourceware.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: H.J. Lu <hjl.tools@gmail.com>
    Cc: Jan Beulich <JBeulich@suse.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/ac027962af343b0c599cbfcf50b945ad2ef3d7a8.1432336324.git.luto@kernel.orgSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    [ luis: backported to 3.16: adjusted context ]
    Signed-off-by: default avatarLuis Henriques <luis.henriques@canonical.com>
    dd727f01
segment.h 8.22 KB