• Josh Poimboeuf's avatar
    objtool: Assume unannotated UD2 instructions are dead ends · 649ea4d5
    Josh Poimboeuf authored
    Arnd reported some false positive warnings with GCC 7:
    
      drivers/hid/wacom_wac.o: warning: objtool: wacom_bpt3_touch()+0x2a5: stack state mismatch: cfa1=7+8 cfa2=6+16
      drivers/iio/adc/vf610_adc.o: warning: objtool: vf610_adc_calculate_rates() falls through to next function vf610_adc_sample_set()
      drivers/pwm/pwm-hibvt.o: warning: objtool: hibvt_pwm_get_state() falls through to next function hibvt_pwm_remove()
      drivers/pwm/pwm-mediatek.o: warning: objtool: mtk_pwm_config() falls through to next function mtk_pwm_enable()
      drivers/spi/spi-bcm2835.o: warning: objtool: .text: unexpected end of section
      drivers/spi/spi-bcm2835aux.o: warning: objtool: .text: unexpected end of section
      drivers/watchdog/digicolor_wdt.o: warning: objtool: dc_wdt_get_timeleft() falls through to next function dc_wdt_restart()
    
    When GCC 7 detects a potential divide-by-zero condition, it sometimes
    inserts a UD2 instruction for the case where the divisor is zero,
    instead of letting the hardware trap on the divide instruction.
    
    Objtool doesn't consider UD2 to be fatal unless it's annotated with
    unreachable().  So it considers the GCC-generated UD2 to be non-fatal,
    and it tries to follow the control flow past the UD2 and gets
    confused.
    
    Previously, objtool *did* assume UD2 was always a dead end.  That
    changed with the following commit:
    
      d1091c7f ("objtool: Improve detection of BUG() and other dead ends")
    
    The motivation behind that change was that Peter was planning on using
    UD2 for __WARN(), which is *not* a dead end.  However, it turns out
    that some emulators rely on UD2 being fatal, so he ended up using
    'ud0' instead:
    
      9a93848f ("x86/debug: Implement __WARN() using UD0")
    
    For GCC 4.5+, it should be safe to go back to the previous assumption
    that UD2 is fatal, even when it's not annotated with unreachable().
    
    But for pre-4.5 versions of GCC, the unreachable() macro isn't
    supported, so such cases of UD2 need to be explicitly annotated as
    reachable.
    Reported-by: default avatarArnd Bergmann <arnd@arndb.de>
    Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Fixes: d1091c7f ("objtool: Improve detection of BUG() and other dead ends")
    Link: http://lkml.kernel.org/r/e57fa9dfede25f79487da8126ee9cdf7b856db65.1501188854.git.jpoimboe@redhat.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    649ea4d5