UBSAN: array-index-out-of-bounds in arch/powerpc/kernel/legacy_serial.c:359:56

Segher Boessenkool segher at
Sat May 8 06:59:28 AEST 2021

On Fri, May 07, 2021 at 10:31:42AM +0200, Christophe Leroy wrote:
> The function is as follows, so when legacy_serial_console == -1 as in your 
> situation, the pointers are just not used.

And it is still undefined behaviour.  C11 6.5.6/8 has
  If both the pointer operand and the result point to elements of the
  same array object, or one past the last element of the array object,
  the evaluation shall not produce an overflow; otherwise, the behavior
  is undefined.
(this is for adding an integer to a pointer).

> When I look into the generated code (UBSAN not selected), we see the 
> verification and the bail-out is done prior to any calculation based on 
> legacy_serial_console.

Yes, you got lucky.  Generating the code you wanted is one of the things
the compiler is allowed to do for UB.

> So, is it normal that UBSAN reports an error here ?

Yes.  It detected undefined behaviour just fine, it did exactly its
job :-)


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