Functions with weak attribute sometimes don't get built into executables if built as a static library

[leventev]

Description

When mlibc is built as a static library functions that are marked as weak with [[gnu::weak]] sometimes aren't included in the final executable. Namely I implemented sys_ttyname and sys_isatty in sysdeps/rook/generic/posix.cpp, both functions are declared in options/posix/include/mlibc/posix-sysdeps.h. Running the final executable causes MLIBC_CHECK_OR_ENOSYS to fail on line 764 in options/posix/generic/unistd-stubs.cpp because the mlibc::sys_ttyname function is not found meaning the weak symbol is undefined. If I move the function definitions to sysdeps/rook/generic/generic.cpp the executable is linked properly and MLIBC_CHECK_OR_ENOSYS does not fail.

Symbols and build file

symbols when functions are defined in posix.cpp:

$ nm root/bin/bash | grep ttyname
00000000004da6aa T ttyname
00000000004da76a T ttyname_r
0000000000000084 b _ZZ7ttynameE3buf
$ nm root/usr/lib/libc.a | grep ttyname
000000000000001b W _ZN5mlibc11sys_ttynameEiPcm
0000000000002d6d T ttyname
0000000000002e2d T ttyname_r
                 w _ZN5mlibc11sys_ttynameEiPcm
0000000000000000 b _ZZ7ttynameE3buf

symbols when functions are defined in generic.cpp:

$ nm root/bin/bash | grep ttyname
00000000004da6aa T ttyname
00000000004da76a T ttyname_r
000000000051f6b6 W _ZN5mlibc11sys_ttynameEiPcm
0000000000000084 b _ZZ7ttynameE3buf
$ nm root/usr/lib/libc.a | grep ttyname
0000000000000276 W _ZN5mlibc11sys_ttynameEiPcm
0000000000002d6d T ttyname
0000000000002e2d T ttyname_r
                 w _ZN5mlibc11sys_ttynameEiPcm
0000000000000000 b _ZZ7ttynameE3buf

meson.build

libc_sources += files(
	'generic/generic.cpp',
	'generic/entry.cpp',
	'generic/posix.cpp'
)

if not no_headers
	install_headers(
		'include/abi-bits/seek-whence.h',
		'include/abi-bits/vm-flags.h',
		'include/abi-bits/errno.h',
		'include/abi-bits/fcntl.h',
		'include/abi-bits/in.h',
		'include/abi-bits/resource.h',
		'include/abi-bits/signal.h',
		'include/abi-bits/stat.h',
		'include/abi-bits/socket.h',
		'include/abi-bits/termios.h',
		'include/abi-bits/time.h',
		'include/abi-bits/blkcnt_t.h',
		'include/abi-bits/blksize_t.h',
		'include/abi-bits/dev_t.h',
		'include/abi-bits/gid_t.h',
		'include/abi-bits/ino_t.h',
		'include/abi-bits/mode_t.h',
		'include/abi-bits/nlink_t.h',
		'include/abi-bits/pid_t.h',
		'include/abi-bits/uid_t.h',
		'include/abi-bits/access.h',
		'include/abi-bits/wait.h',
		'include/abi-bits/limits.h',
		'include/abi-bits/utsname.h',
		'include/abi-bits/ptrace.h',
		'include/abi-bits/poll.h',
		'include/abi-bits/epoll.h',
		'include/abi-bits/packet.h',
		'include/abi-bits/inotify.h',
		'include/abi-bits/clockid_t.h',
		'include/abi-bits/shm.h',
		'include/abi-bits/mqueue.h',
		'include/abi-bits/suseconds_t.h',
		'include/abi-bits/fsfilcnt_t.h',
		'include/abi-bits/fsblkcnt_t.h',
		'include/abi-bits/socklen_t.h',
		'include/abi-bits/statfs.h',
		'include/abi-bits/statvfs.h',
		'include/abi-bits/ioctls.h',
		'include/abi-bits/xattr.h',
		subdir: 'abi-bits'
	)
	install_headers(
		'include/rook/syscall.h',
		'include/rook/syscalls.h',
		subdir: 'rook'
	)
endif

if not headers_only
	crt0 = custom_target('crt0',
		build_by_default: true,
		command: c_compiler.cmd_array() + ['-c', '-o', '@OUTPUT@', '@INPUT@'],
		input: 'crt-x86_64/crt0.S',
		output: 'crt0.o',
		install: true,
		install_dir: get_option('libdir')
	)
endif

if host_machine.cpu_family() != 'x86_64'
	error('Unknown architecture')
endif

Details

• changing the order of the files in libc_sources didn't work
• both files include the exact same headers in the exact same order
• the functions were in the right namespace in both files and their function signatures, mangled symbol names matched which can be confirmed by comparing the outputs of nm which i provided above
• the function body is irrelevant, whether it was a stub or the function was actually implemented didn't make a difference
• in both cases the right symbols exist in the static library but at different addresses

Temporary solutions

• removing the '[[gnu:weak]]' attribute from the 2 function declarations in the header file thus changing it to a strong symbol
• putting all function definitions in the same source file

[moodyhunter]

same issue here, I ended up removing weak attributes manually for failing sysdeps

[64]

Not really sure what causes this -- can anyone provide a reproducer?