Since commit feaf756587 ("nolibc: fix fd_set type"), an fd_set is an
array of unsigned long, but the FD_* macros still build their masks as
an int or an unsigned int: 1 << n in FD_SET(), 1U << n in FD_CLR() and
FD_ISSET(), where n is the fd modulo 64 on 64-bit architectures. So:
- FD_SET() of an fd with n == 31 sets the bits for n = 31-63, as
1 << 31 is negative and gets sign-extended.
- For n >= 32 the shifts are undefined. x86-64, for instance, masks
the shift count, so FD_SET(40) and FD_ISSET(40) use the bit of fd 8.
- FD_CLR() zero-extends ~(1U << n), so it also clears the bits for
n = 32-63, whichever fd it is given.
select() on fd 40, for instance, fails with EBADF, or watches fd 8
instead if that one is open.
Build the masks from 1UL.
Fixes: feaf756587 ("nolibc: fix fd_set type")
Assisted-by: LLM
Signed-off-by: Danish Khateeb <danishkhateeb03@gmail.com>
Link: https://patch.msgid.link/20260925211600.119116-3-danishkhateeb03@gmail.com
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
929ea5b348