[ABI BREAK]: abis: Fix definition of fd_set

Author: Dennisbonke

ABI_BREAKS.md

@@ -27,6 +27,7 @@ This document lists the ABI breaks that were made in each mlibc major version.
 - [#1460](https://github.com/managarm/mlibc/pull/1460): `useconds_t` was turned into a 32-bit value, as it only needs to hold microsecond values worth up to a second.
 - [#1492](https://github.com/managarm/mlibc/pull/1492): changes the values of `LC_*` macros and `nl_item` values to match glibc, so that glibc locale files can be consumed.
 - [#1492](https://github.com/managarm/mlibc/pull/1492): fix `struct epoll_event` alignment on x86
+- [#1658](https://github.com/managarm/mlibc/pull/1658): fix incorrect definition of `struct fd_set`
 
 ## Version 6
 

abis/ironclad/fd_set.h

@@ -5,8 +5,11 @@
 
 #define FD_SETSIZE 1024
 
+typedef __mlibc_uint8 __fd_mask;
+#define __NFDBITS (8 * (int) sizeof (__fd_mask))
+
 typedef struct __attribute__((__aligned__(__alignof__(long)))) {
-	__mlibc_uint8 fds_bits[128];
+	__fd_mask fds_bits[FD_SETSIZE / __NFDBITS];
 } fd_set;
 
 #endif /* _ABIBITS_FD_SET_H */

abis/linux/fd_set.h

@@ -5,8 +5,13 @@
 
 #define FD_SETSIZE 1024
 
-typedef struct __attribute__((__aligned__(__alignof__(long)))) {
-	__mlibc_uint8 fds_bits[128];
+typedef long int __fd_mask;
+#define __NFDBITS (8 * (int) sizeof (__fd_mask))
+
+typedef struct {
+	/* libtirpc reaches into fds_bits directly, so we must keep the name and type exactly as expected.
+	 * This now matches the definition in sys/select.h from glibc. */
+	__fd_mask fds_bits[FD_SETSIZE / __NFDBITS];
 } fd_set;
 
 #endif /* _ABIBITS_FD_SET_H */

options/posix/generic/sys-select.cpp

@@ -1,3 +1,5 @@
+#include <bit>
+#include <type_traits>
 
 #include <string.h>
 #include <sys/select.h>
@@ -9,17 +11,21 @@
 
 #include <mlibc/posix-sysdeps.hpp>
 
+using fd_mask_unsigned = std::make_unsigned_t<__fd_mask>;
+
+#define FD_MASK(n) (fd_mask_unsigned{1} << (n))
+
 void __FD_CLR(int fd, fd_set *set) {
-	__ensure(fd < FD_SETSIZE);
-	set->fds_bits[fd / 8] &= ~(1 << (fd % 8));
+    __ensure(fd < FD_SETSIZE);
+    set->fds_bits[fd / NFDBITS] &= std::bit_cast<fd_mask>(~FD_MASK(fd % NFDBITS));
 }
 int __FD_ISSET(int fd, fd_set *set) {
-	__ensure(fd < FD_SETSIZE);
-	return set->fds_bits[fd / 8] & (1 << (fd % 8));
+    __ensure(fd < FD_SETSIZE);
+    return (std::bit_cast<fd_mask_unsigned>(set->fds_bits[fd / NFDBITS]) & FD_MASK(fd % NFDBITS)) != 0;
 }
 void __FD_SET(int fd, fd_set *set) {
-	__ensure(fd < FD_SETSIZE);
-	set->fds_bits[fd / 8] |= 1 << (fd % 8);
+    __ensure(fd < FD_SETSIZE);
+    set->fds_bits[fd / NFDBITS] = std::bit_cast<fd_mask>(std::bit_cast<fd_mask_unsigned>(set->fds_bits[fd / NFDBITS]) | FD_MASK(fd % NFDBITS));
 }
 void __FD_ZERO(fd_set *set) {
 	memset(set->fds_bits, 0, sizeof(fd_set));

options/posix/include/sys/select.h

@@ -14,9 +14,6 @@
 extern "C" {
 #endif
 
-typedef long int __fd_mask;
-#define __NFDBITS (8 * (int) sizeof (__fd_mask))
-
 #if defined(_DEFAULT_SOURCE)
 typedef __fd_mask fd_mask;
 #define NFDBITS __NFDBITS