Re: Clang UndefinedBehaviorSanitize (Postgres14) Detected undefined-behavior

On Sat, Aug 29, 2020 at 12:36:42PM -0400, Tom Lane wrote:
> Peter Geoghegan <pg@bowt.ie> writes:
> > I wonder if we should start using -fno-delete-null-pointer-checks:
> > https://lkml.org/lkml/2018/4/4/601
> > This may not be strictly relevant to the discussion, but I was
> > reminded of it just now and thought I'd mention it.
> 
> Hmm.  gcc 8.3 defines this as:
> 
>      Assume that programs cannot safely dereference null pointers, and
>      that no code or data element resides at address zero.  This option
>      enables simple constant folding optimizations at all optimization
>      levels.  In addition, other optimization passes in GCC use this
>      flag to control global dataflow analyses that eliminate useless
>      checks for null pointers; these assume that a memory access to
>      address zero always results in a trap, so that if a pointer is
>      checked after it has already been dereferenced, it cannot be null.
> 
> AFAICS, that's a perfectly valid assumption for our usage.  I can see why
> the kernel might not want it, but we set things up whenever possible to
> ensure that dereferencing NULL would crash.

We do assume dereferencing NULL would crash, but we also assume this
optimization doesn't happen:

=== opt-null.c
#include <string.h>
#include <unistd.h>

int my_memcpy(void *dest, const void *src, size_t n)
{
#ifndef REMOVE_MEMCPY
    memcpy(dest, src, n);
#endif
    if (src)
	pause();
    return 0;
}
===

$ gcc --version
gcc (Debian 8.3.0-6) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ diff -sU1 <(gcc -O2 -fno-delete-null-pointer-checks -S -o- opt-null.c) <(gcc -O2 -S -o- opt-null.c)
--- /dev/fd/63  2020-09-03 19:23:53.206864378 -0700
+++ /dev/fd/62  2020-09-03 19:23:53.206864378 -0700
@@ -8,13 +8,8 @@
        .cfi_startproc
-       pushq   %rbx
+       subq    $8, %rsp
        .cfi_def_cfa_offset 16
-       .cfi_offset 3, -16
-       movq    %rsi, %rbx
        call    memcpy@PLT
-       testq   %rbx, %rbx
-       je      .L2
        call    pause@PLT
-.L2:
        xorl    %eax, %eax
-       popq    %rbx
+       addq    $8, %rsp
        .cfi_def_cfa_offset 8
$ diff -sU1 <(gcc -DREMOVE_MEMCPY -O2 -fno-delete-null-pointer-checks -S -o- opt-null.c) <(gcc -DREMOVE_MEMCPY -O2 -S -o- opt-null.c)
Files /dev/fd/63 and /dev/fd/62 are identical


So yes, it would be reasonable to adopt -fno-delete-null-pointer-checks and/or
remove -fno-sanitize=nonnull-attribute from buildfarm member thorntail.

> However, while grepping the manual for that I also found
> 
> '-Wnull-dereference'
>      Warn if the compiler detects paths that trigger erroneous or
>      undefined behavior due to dereferencing a null pointer.  This
>      option is only active when '-fdelete-null-pointer-checks' is
>      active, which is enabled by optimizations in most targets.  The
>      precision of the warnings depends on the optimization options used.
> 
> I wonder whether turning that on would find anything interesting.

Promising.  Sadly, it doesn't warn for the above test case.