Reimplement brk() and sbrk() to avoid the use of _end.

Previously, libc.so would initialize its notion of the break address
using _end, a special symbol emitted by the static linker following
the bss section.  Compatibility issues between lld and ld.bfd could
cause the wrong definition of _end (libc.so's definition rather than
that of the executable) to be used, breaking the brk()/sbrk()
interface.

Avoid this problem and future interoperability issues by simply not
relying on _end.  Instead, modify the break() system call to return
the kernel's view of the current break address, and have libc
initialize its state using an extra syscall upon the first use of the
interface.  As a side effect, this appears to fix brk()/sbrk() usage
in executables run with rtld direct exec, since the kernel and libc.so
no longer maintain separate views of the process' break address.

PR:		228574
Reviewed by:	kib (previous version)
MFC after:	2 months
Differential Revision:	https://reviews.freebsd.org/D15663

1 files changed, 10 insertions, 4 deletions

diff --git a/sys/vm/vm_unix.c b/sys/vm/vm_unix.c
index b53e6fb6614e..936223629231 100644
--- a/sys/vm/vm_unix.c
+++ b/sys/vm/vm_unix.c
@@ -102,13 +102,16 @@ sys_obreak(struct thread *td, struct obreak_args *uap)
 		}
 	} else if (new < base) {
 		/*
-		 * This is simply an invalid value.  If someone wants to
-		 * do fancy address space manipulations, mmap and munmap
-		 * can do most of what the user would want.
+		 * Simply return the current break address without
+		 * modifying any state.  This is an ad-hoc interface
+		 * used by libc to determine the initial break address,
+		 * avoiding a dependency on magic features in the system
+		 * linker.
 		 */
-		error = EINVAL;
+		new = old;
 		goto done;
 	}
+
 	if (new > old) {
 		if (!old_mlock && map->flags & MAP_WIREFUTURE) {
 			if (ptoa(pmap_wired_count(map->pmap)) +
@@ -225,6 +228,9 @@ done:
 		(void) vm_map_wire(map, old, new,
 		    VM_MAP_WIRE_USER|VM_MAP_WIRE_NOHOLES);
 
+	if (error == 0)
+		td->td_retval[0] = new;
+
 	return (error);
 #else /* defined(__aarch64__) || defined(__riscv__) */
 	return (ENOSYS);