Re: [RFC][PATCH] Speed-up use-after-scope (re-writing to SSA)

["Martin Liška" <mliska@suse.cz>]

On 11/16/2016 01:25 PM, Martin Liška wrote:
> Hello
> 
> Following patch is a candidate that re-writes VAR_DECLs that are
> is_gimple_reg_type with:
> my_char_25 = ASAN_POISON ();
> 
> that is eventually transformed to:
> __builtin___asan_report_use_after_scope_noabort ("my_char", 1);
> 
> at places where my_char_25 is used. That introduces a new entry point
> to ASAN runtime, reporting:
> 
> ==18378==ERROR: AddressSanitizer: stack-use-after-scope at pc 0x0000004007b4 bp 0x000000000001 sp 0x000000400603
> ACCESS of size 1 for variable 'my_char' thread T0
>     #0 0x400602 in main (/tmp/a.out+0x400602)
>     #1 0x7fa6e572d290 in __libc_start_main (/lib64/libc.so.6+0x20290)
>     #2 0x400669 in _start (/tmp/a.out+0x400669)
> 
> SUMMARY: AddressSanitizer: stack-use-after-scope (/tmp/a.out+0x400602) in main
> 
> I'm still not sure where exactly do the expansion of ASAN_POISON as some cleanup
> after the transformation would be desired.
> 
> Thoughts?
> Thanks,
> Martin 
> 
> 
> 
> 

There's an example:

int
main (void)
{
  char *ptr;
  {
    char my_char;
    ptr = &my_char;
  }

  return *ptr;
}

$ g++ /tmp/use-after-scope-1.c -fsanitize=address -O0 && ./a.out 
=================================================================
==16035==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7ffe76322240 at pc 0x000000400848 bp 0x7ffe76322200 sp 0x7ffe763221f8
READ of size 1 at 0x7ffe76322240 thread T0
    #0 0x400847 in main (/tmp/a.out+0x400847)
    #1 0x7f0005739290 in __libc_start_main (/lib64/libc.so.6+0x20290)
    #2 0x4006b9 in _start (/tmp/a.out+0x4006b9)

Address 0x7ffe76322240 is located in stack of thread T0 at offset 32 in frame
    #0 0x400786 in main (/tmp/a.out+0x400786)

  This frame has 1 object(s):
    [32, 33) 'my_char' <== Memory access at offset 32 is inside this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-use-after-scope (/tmp/a.out+0x400847) in main
Shadow bytes around the buggy address:
  0x10004ec5c3f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004ec5c400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004ec5c410: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004ec5c420: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004ec5c430: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x10004ec5c440: 00 00 00 00 f1 f1 f1 f1[f8]f2 f2 f2 f3 f3 f3 f3
  0x10004ec5c450: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004ec5c460: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004ec5c470: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004ec5c480: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004ec5c490: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==16035==ABORTING

$ g++ /tmp/use-after-scope-1.c -fsanitize=address -O2 && ./a.out 
=================================================================
==16049==ERROR: AddressSanitizer: stack-use-after-scope at pc 0x000000400794 bp 0x000000000001 sp 0x0000004005f3
ACCESS of size 1 for variable 'my_char' thread T0
    #0 0x4005f2 in main (/tmp/a.out+0x4005f2)
    #1 0x7f883337e290 in __libc_start_main (/lib64/libc.so.6+0x20290)
    #2 0x400649 in _start (/tmp/a.out+0x400649)

SUMMARY: AddressSanitizer: stack-use-after-scope (/tmp/a.out+0x4005f2) in main
==16049==ABORTING

Martin