Re: [PATCH 0/2] topic: meson: add more compiler hardening flags

[Thomas Huth <thuth@redhat.com>]

On 05/10/2023 19.38, Daniel P. Berrangé wrote:
...
> 
> I also tested enabling -ftrapv, to change signed integer
> overflow from wrapping, to trapping instead. This exposed a
> bug in the string-input-visitor which overflows when parsing
> ranges, and exposed the test-int128 code as (harmlessly)
> overflowing during its testing. Both can be fixed, but I'm
> not entirely sure whether -ftrapv is viable or not. I was
> wondering about TCG and whether it has a need to intentionally
> allow integer overflow for any of its instruction emulation
> requirements ?
I'm not an expert when it comes to this question, but as far as I 
understood, we are using -fwrapv (with "w", not "t") on purpose, see 
meson.build:

# We use -fwrapv to tell the compiler that we require a C dialect where
# left shift of signed integers is well defined and has the expected
# 2s-complement style results. (Both clang and gcc agree that it
# provides these semantics.)

And according to the man-page of gcc:

  The options -ftrapv and -fwrapv override each other,
  so using -ftrapv -fwrapv on the command-line results
  in -fwrapv being effective.

If I got that right, this means you cannot use -ftrapv with QEMU.

  Thomas