--werror doesn't work with -Db_lto=true

[evverx]

It appears -Werror gets lost somewhere along the way when -Db_lto is used. For example, systemd/systemd#16224 was discovered by passing -Werror with CFLAGS directly.

Here's what the command line looks like with --werror:

[1053/1636] cc  -o systemd-networkd 'systemd-networkd@exe/src_network_networkd.c.o' -flto -Wl,--as-needed -Wl,--no-undefined -Wl,-z,relro -Wl,-z,now -fstack-protector -Wl,--gc-sections -Wl,--start-group src/network/libnetworkd-core.a src/shared/libsystemd-shared-245.so src/libsystemd-network/libsystemd-network.a src/udev/libudev_static.a src/udev/libudev-basic.a -Wl,--end-group -pthread '-Wl,-rpath,$ORIGIN/src/network:$ORIGIN/src/shared:$ORIGIN/src/libsystemd-network:$ORIGIN/src/udev' -Wl,-rpath-link,/home/vagrant/systemd/build/src/network -Wl,-rpath-link,/home/vagrant/systemd/build/src/shared -Wl,-rpath-link,/home/vagrant/systemd/build/src/libsystemd-network -Wl,-rpath-link,/home/vagrant/systemd/build/src/udev
In function ‘dhcp6_assign_delegated_prefix’,
    inlined from ‘dhcp6_pd_prefix_assign’ at ../src/network/networkd-dhcp6.c:203:21,
    inlined from ‘dhcp6_pd_prefix_distribute.isra’ at ../src/network/networkd-dhcp6.c:333:21:
../src/network/networkd-dhcp6.c:1068:17: warning: writing 8 bytes into a region of size 0 [-Wstringop-overflow=]
 1068 |                 memcpy(&address->in_addr.in6.s6_addr + 8, &link->network->dhcp6_delegation_prefix_token.in6.s6_addr + 8, 8);
      |                 ^
../src/network/networkd-dhcp6.c: In function ‘dhcp6_pd_prefix_distribute.isra’:
../src/basic/alloc-util.h:96:16: note: at offset 196 to an object with size 0 allocated by ‘malloc’ here
   96 |         return malloc(size * need ?: 1);
      |                ^
[1054/1636]

and with CFLAGS='-Werror'

2020-06-19T23:14:53.6541938Z gcc-10  -o systemd-networkd 'systemd-networkd@exe/src_network_networkd.c.o' -flto -Wl,--as-needed -Wl,--no-undefined -Wl,-z,relro -Wl,-z,now -fstack-protector -Wl,--gc-sections -Werror -Wl,--start-group src/network/libnetworkd-core.a src/shared/libsystemd-shared-245.so src/libsystemd-network/libsystemd-network.a src/udev/libudev_static.a src/udev/libudev-basic.a -Wl,--end-group -pthread '-Wl,-rpath,$ORIGIN/src/network:$ORIGIN/src/shared:$ORIGIN/src/libsystemd-network:$ORIGIN/src/udev' -Wl,-rpath-link,/home/runner/work/systemd/systemd/build/src/network -Wl,-rpath-link,/home/runner/work/systemd/systemd/build/src/shared -Wl,-rpath-link,/home/runner/work/systemd/systemd/build/src/libsystemd-network -Wl,-rpath-link,/home/runner/work/systemd/systemd/build/src/udev
2020-06-19T23:14:53.6542368Z In function ‘memcpy’,
2020-06-19T23:14:53.6542707Z     inlined from ‘dhcp6_assign_delegated_prefix’ at ../src/network/networkd-dhcp6.c:1068:17,
2020-06-19T23:14:53.6543056Z     inlined from ‘dhcp6_pd_prefix_assign’ at ../src/network/networkd-dhcp6.c:203:21,
2020-06-19T23:14:53.6543409Z     inlined from ‘dhcp6_pd_prefix_distribute.isra’ at ../src/network/networkd-dhcp6.c:333:21:
2020-06-19T23:14:53.6543809Z /usr/include/x86_64-linux-gnu/bits/string_fortified.h:34:10: error: writing 8 bytes into a region of size 0 [-Werror=stringop-overflow=]
2020-06-19T23:14:53.6544264Z    34 |   return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
2020-06-19T23:14:53.6544409Z       |          ^
2020-06-19T23:14:53.6544737Z ../src/network/networkd-dhcp6.c: In function ‘dhcp6_pd_prefix_distribute.isra’:
2020-06-19T23:14:53.6545096Z ../src/basic/alloc-util.h:96:16: note: at offset 196 to an object with size 0 allocated by ‘malloc’ here
2020-06-19T23:14:53.6545230Z    96 |         return malloc(size * need ?: 1);
2020-06-19T23:14:53.6545358Z       |                ^
2020-06-19T23:14:53.6545485Z lto1: all warnings being treated as errors
2020-06-19T23:14:53.6545771Z lto-wrapper: fatal error: gcc-10 returned 1 exit status
2020-06-19T23:14:53.6545904Z compilation terminated.
2020-06-19T23:14:53.6546164Z /usr/bin/ld: error: lto-wrapper failed
2020-06-19T23:14:53.6546295Z collect2: error: ld returned 1 exit status

[evverx]

Forgot to mention that it's reproducible with meson-0.54.3. And just in case the full log can be found at https://github.com/systemd/systemd/runs/789680623?check_suite_focus=true (where -Werror was passed with CFLAGS) and at https://github.com/systemd/systemd/runs/789365442?check_suite_focus=true (where --werror was used).

[evverx]

It appears there is no way to treat warnings as errors using meson. --werror doesn't always work. -Werror passed via CFLAGS or -Dc_args can break compiler checks leading to meson failing to find "deprecated" functions with cc.has_function: systemd/systemd#18439