Support syscall fchmodat2

book/src/kernel/linux-compatibility/README.md

@@ -288,7 +288,7 @@ which are summarized in the table below.
 | 265     | linkat                 | ✅             | ❓ |
 | 266     | symlinkat              | ✅             | 💯 |
 | 267     | readlinkat             | ✅             | 💯 |
-| 268     | fchmodat               | ✅             | ❓ |
+| 268     | fchmodat               | ✅             | 💯 |
 | 269     | faccessat              | ✅             | ❓ |
 | 270     | pselect6               | ✅             | 💯 |
 | 271     | ppoll                  | ✅             | ❓ |
@@ -348,6 +348,7 @@ which are summarized in the table below.
 | 436     | close_range            | ✅             | ❓ |
 | 439     | faccessat2             | ✅             | ❓ |
 | 441     | epoll_pwait2           | ✅             | ❓ |
+| 452     | fchmodat2              | ✅             | 💯 |
 
 - Supported:
     - ✅ = syscall supported

book/src/kernel/linux-compatibility/syscall-flag-coverage/file-and-directory-operations/fully_covered.scml

@@ -68,6 +68,8 @@ readlinkat(dirfd, path, buf, bufsiz);
 // Change permissions of a file
 chmod(path, mode);
 fchmod(fd, mode);
+fchmodat(dirfd, path_ptr, mode);
+fchmodat2(dirfd, path_ptr, mode, flags = AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW);
 
 // Change ownership of a file
 chown(path, owner, group);

kernel/src/syscall/arch/loongarch.rs

@@ -10,7 +10,7 @@ use super::{
     capget::sys_capget,
     capset::sys_capset,
     chdir::{sys_chdir, sys_fchdir},
-    chmod::{sys_fchmod, sys_fchmodat},
+    chmod::{sys_fchmod, sys_fchmodat, sys_fchmodat2},
     chown::{sys_fchown, sys_fchownat},
     chroot::sys_chroot,
     clock_gettime::sys_clock_gettime,
@@ -356,4 +356,5 @@ impl_syscall_nums_and_dispatch_fn! {
     SYS_CLOSE_RANGE = 436            => sys_close_range(args[..3]);
     SYS_FACCESSAT2 = 439             => sys_faccessat2(args[..4]);
     SYS_EPOLL_PWAIT2 = 441           => sys_epoll_pwait2(args[..5]);
+    SYS_FCHMODAT2 = 452              => sys_fchmodat2(args[..4]);
 }

kernel/src/syscall/arch/riscv.rs

@@ -10,7 +10,7 @@ use super::{
     capget::sys_capget,
     capset::sys_capset,
     chdir::{sys_chdir, sys_fchdir},
-    chmod::{sys_fchmod, sys_fchmodat},
+    chmod::{sys_fchmod, sys_fchmodat, sys_fchmodat2},
     chown::{sys_fchown, sys_fchownat},
     chroot::sys_chroot,
     clock_gettime::sys_clock_gettime,
@@ -358,4 +358,5 @@ impl_syscall_nums_and_dispatch_fn! {
     SYS_CLOSE_RANGE = 436            => sys_close_range(args[..3]);
     SYS_FACCESSAT2 = 439             => sys_faccessat2(args[..4]);
     SYS_EPOLL_PWAIT2 = 441           => sys_epoll_pwait2(args[..5]);
+    SYS_FCHMODAT2 = 452              => sys_fchmodat2(args[..4]);
 }

kernel/src/syscall/arch/x86.rs

@@ -12,7 +12,7 @@ use super::{
     capget::sys_capget,
     capset::sys_capset,
     chdir::{sys_chdir, sys_fchdir},
-    chmod::{sys_chmod, sys_fchmod, sys_fchmodat},
+    chmod::{sys_chmod, sys_fchmod, sys_fchmodat, sys_fchmodat2},
     chown::{sys_chown, sys_fchown, sys_fchownat, sys_lchown},
     chroot::sys_chroot,
     clock_gettime::sys_clock_gettime,
@@ -407,4 +407,5 @@ impl_syscall_nums_and_dispatch_fn! {
     SYS_CLOSE_RANGE = 436      => sys_close_range(args[..3]);
     SYS_FACCESSAT2 = 439       => sys_faccessat2(args[..4]);
     SYS_EPOLL_PWAIT2 = 441     => sys_epoll_pwait2(args[..5]);
+    SYS_FCHMODAT2 = 452        => sys_fchmodat2(args[..4]);
 }

kernel/src/syscall/chmod.rs

@@ -24,32 +24,62 @@ pub fn sys_fchmod(fd: FileDesc, mode: u16, ctx: &Context) -> Result<SyscallRetur
 }
 
 pub fn sys_chmod(path_ptr: Vaddr, mode: u16, ctx: &Context) -> Result<SyscallReturn> {
-    self::sys_fchmodat(AT_FDCWD, path_ptr, mode, ctx)
+    do_fchmodat(AT_FDCWD, path_ptr, mode, ChmodFlags::empty(), ctx)
 }
 
-// Glibc handles the `flags` argument, so we just ignore it.
 pub fn sys_fchmodat(
     dirfd: FileDesc,
     path_ptr: Vaddr,
     mode: u16,
-    /* flags: u32, */
+    ctx: &Context,
+) -> Result<SyscallReturn> {
+    do_fchmodat(dirfd, path_ptr, mode, ChmodFlags::empty(), ctx)
+}
+
+pub fn sys_fchmodat2(
+    dirfd: FileDesc,
+    path_ptr: Vaddr,
+    mode: u16,
+    flags: u32,
+    ctx: &Context,
+) -> Result<SyscallReturn> {
+    let flags = ChmodFlags::from_bits(flags)
+        .ok_or_else(|| Error::with_message(Errno::EINVAL, "invalid chmod flags"))?;
+
+    do_fchmodat(dirfd, path_ptr, mode, flags, ctx)
+}
+
+fn do_fchmodat(
+    dirfd: FileDesc,
+    path_ptr: Vaddr,
+    mode: u16,
+    flags: ChmodFlags,
     ctx: &Context,
 ) -> Result<SyscallReturn> {
     let path_name = ctx.user_space().read_cstring(path_ptr, PATH_MAX)?;
+
     debug!(
-        "dirfd = {}, path_name = {:?}, mode = 0o{:o}",
-        dirfd, path_name, mode,
+        "dirfd = {}, path_name = {:?}, mode = 0o{:o}, flags = {:?}",
+        dirfd, path_name, mode, flags,
     );
 
     let path_or_inode = {
         let path_name = path_name.to_string_lossy();
-        let fs_path = FsPath::from_fd_and_path(dirfd, &path_name)?;
-        ctx.thread_local
-            .borrow_fs()
-            .resolver()
-            .read()
-            .lookup_inode(&fs_path)?
+        let fs_path = if flags.contains(ChmodFlags::AT_EMPTY_PATH) && path_name.is_empty() {
+            FsPath::from_fd(dirfd)?
+        } else {
+            FsPath::from_fd_and_path(dirfd, &path_name)?
+        };
+
+        let fs_ref = ctx.thread_local.borrow_fs();
+        let fs = fs_ref.resolver().read();
+        if flags.contains(ChmodFlags::AT_SYMLINK_NOFOLLOW) {
+            fs.lookup_inode_no_follow(&fs_path)?
+        } else {
+            fs.lookup_inode(&fs_path)?
+        }
     };
+
     path_or_inode
         .inode()
         .set_mode(InodeMode::from_bits_truncate(mode))?;
@@ -58,3 +88,10 @@ pub fn sys_fchmodat(
     }
     Ok(SyscallReturn::Return(0))
 }
+
+bitflags::bitflags! {
+    struct ChmodFlags: u32 {
+        const AT_EMPTY_PATH = 1 << 12;
+        const AT_SYMLINK_NOFOLLOW = 1 << 8;
+    }
+}