Kernel+UE: Add MAP_FIXED_NOREPLACE mmap() flag

This feature was introduced in version 4.17 of the Linux kernel, and
while it's not specified by POSIX, I think it will be a nice addition to
our system.

MAP_FIXED_NOREPLACE provides a less error-prone alternative to
MAP_FIXED: while regular fixed mappings would cause any intersecting
ranges to be unmapped, MAP_FIXED_NOREPLACE returns EEXIST instead. This
ensures that we don't corrupt our process's address space if something
is already at the requested address.

Note that the more portable way to do this is to use regular
MAP_ANONYMOUS, and check afterwards whether the returned address matches
what we wanted. This, however, has a large performance impact on
programs like Wine which try to reserve large portions of the address
space at once, as the non-matching addresses have to be unmapped
separately.

Base/usr/share/man/man2/pledge.md

@@ -53,7 +53,7 @@ If the process later attempts to use any system functionality it has previously
 * `recvfd`: Receive file descriptors over a local socket
 * `ptrace`: The [`ptrace`(2)](ptrace.md) syscall (\*)
 * `prot_exec`: [`mmap`(2)](mmap.md) and [`mprotect`(2)](mprotect.md) with `PROT_EXEC`
-* `map_fixed`: [`mmap`(2)](mmap.md) with `MAP_FIXED` (\*)
+* `map_fixed`: [`mmap`(2)](mmap.md) with `MAP_FIXED` or `MAP_FIXED_NOREPLACE` (\*)
 
 Promises marked with an asterisk (\*) are SerenityOS specific extensions not supported by the original OpenBSD `pledge()`.
 

Kernel/API/POSIX/sys/mman.h

@@ -23,6 +23,7 @@ extern "C" {
 #define MAP_NORESERVE 0x80
 #define MAP_RANDOMIZED 0x100
 #define MAP_PURGEABLE 0x200
+#define MAP_FIXED_NOREPLACE 0x400
 
 #define PROT_READ 0x1
 #define PROT_WRITE 0x2

Kernel/Syscalls/mmap.cpp

@@ -135,7 +135,7 @@ ErrorOr<FlatPtr> Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> use
         REQUIRE_PROMISE(prot_exec);
     }
 
-    if (prot & MAP_FIXED) {
+    if (prot & MAP_FIXED || prot & MAP_FIXED_NOREPLACE) {
         REQUIRE_PROMISE(map_fixed);
     }
 
@@ -167,6 +167,7 @@ ErrorOr<FlatPtr> Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> use
     bool map_fixed = flags & MAP_FIXED;
     bool map_noreserve = flags & MAP_NORESERVE;
     bool map_randomized = flags & MAP_RANDOMIZED;
+    bool map_fixed_noreplace = flags & MAP_FIXED_NOREPLACE;
 
     if (map_shared && map_private)
         return EINVAL;
@@ -174,7 +175,7 @@ ErrorOr<FlatPtr> Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> use
     if (!map_shared && !map_private)
         return EINVAL;
 
-    if (map_fixed && map_randomized)
+    if ((map_fixed || map_fixed_noreplace) && map_randomized)
         return EINVAL;
 
     if (!validate_mmap_prot(prot, map_stack, map_anonymous))
@@ -186,19 +187,18 @@ ErrorOr<FlatPtr> Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> use
     Memory::Region* region = nullptr;
 
     auto range = TRY([&]() -> ErrorOr<Memory::VirtualRange> {
-        if (map_randomized) {
+        if (map_randomized)
             return address_space().page_directory().range_allocator().try_allocate_randomized(Memory::page_round_up(size), alignment);
-        }
+
+        // If MAP_FIXED is specified, existing mappings that intersect the requested range are removed.
+        if (map_fixed)
+            TRY(address_space().unmap_mmap_range(VirtualAddress(addr), size));
 
         auto range = address_space().try_allocate_range(VirtualAddress(addr), size, alignment);
         if (range.is_error()) {
-            if (addr && !map_fixed) {
+            if (addr && !(map_fixed || map_fixed_noreplace)) {
                 // If there's an address but MAP_FIXED wasn't specified, the address is just a hint.
                 range = address_space().try_allocate_range({}, size, alignment);
-            } else if (map_fixed) {
-                // If MAP_FIXED is specified, existing mappings that intersect the requested range are removed.
-                TRY(address_space().unmap_mmap_range(VirtualAddress(addr), size));
-                range = address_space().try_allocate_range(VirtualAddress(addr), size, alignment);
             }
         }
         return range;

Userland/DevTools/UserspaceEmulator/Emulator_syscalls.cpp

@@ -873,10 +873,11 @@ u32 Emulator::virt$mmap(u32 params_addr)
     Optional<Range> result;
     if (params.flags & MAP_RANDOMIZED) {
         result = m_range_allocator.allocate_randomized(requested_size, params.alignment);
-    } else if (params.flags & MAP_FIXED) {
+    } else if (params.flags & MAP_FIXED || params.flags & MAP_FIXED_NOREPLACE) {
         if (params.addr) {
             // If MAP_FIXED is specified, existing mappings that intersect the requested range are removed.
-            virt$munmap(params.addr, requested_size);
+            if (params.flags & MAP_FIXED)
+                virt$munmap(params.addr, requested_size);
             result = m_range_allocator.allocate_specific(VirtualAddress { params.addr }, requested_size);
         } else {
             // mmap(nullptr, …, MAP_FIXED) is technically okay, but tends to be a bug.

Userland/DevTools/UserspaceEmulator/MmapRegion.cpp

@@ -36,8 +36,8 @@ NonnullOwnPtr<MmapRegion> MmapRegion::create_anonymous(u32 base, u32 size, u32 p
 
 NonnullOwnPtr<MmapRegion> MmapRegion::create_file_backed(u32 base, u32 size, u32 prot, int flags, int fd, off_t offset, String name)
 {
-    // Since we put the memory to an arbitrary location, do not pass MAP_FIXED to the Kernel.
+    // Since we put the memory to an arbitrary location, do not pass MAP_FIXED and MAP_FIXED_NOREPLACE to the Kernel.
-    auto real_flags = flags & ~MAP_FIXED;
+    auto real_flags = flags & ~(MAP_FIXED | MAP_FIXED_NOREPLACE);
     auto* data = (u8*)mmap_with_name(nullptr, size, prot, real_flags, fd, offset, name.is_empty() ? nullptr : String::formatted("(UE) {}", name).characters());
     VERIFY(data != MAP_FAILED);
     auto* shadow_data = (u8*)mmap_initialized(size, 1, "MmapRegion ShadowData");

Userland/Utilities/strace.cpp

@@ -634,7 +634,8 @@ static void format_recvmsg(FormattedSyscallBuilder& builder, int socket, struct
 struct MmapFlags : BitflagBase {
     static constexpr auto options = {
         BITFLAG(MAP_SHARED), BITFLAG(MAP_PRIVATE), BITFLAG(MAP_FIXED), BITFLAG(MAP_ANONYMOUS),
-        BITFLAG(MAP_RANDOMIZED), BITFLAG(MAP_STACK), BITFLAG(MAP_NORESERVE), BITFLAG(MAP_PURGEABLE)
+        BITFLAG(MAP_RANDOMIZED), BITFLAG(MAP_STACK), BITFLAG(MAP_NORESERVE), BITFLAG(MAP_PURGEABLE),
+        BITFLAG(MAP_FIXED_NOREPLACE)
     };
     static constexpr StringView default_ = "MAP_FILE";
 };