This commit adds support for the `openat2()` syscall in the `linux-user` userspace emulator.
It is implemented by extracting a new helper `maybe_do_fake_open()` out of the exiting `do_guest_openat()` and share that with the new `do_guest_openat2()`. Unfortunatly we cannot just make do_guest_openat2() a superset of do_guest_openat() because the openat2() syscall is stricter with the argument checking and will return an error for invalid flags or mode combinations (which open()/openat() will ignore). The implementation is similar to SYSCALL_DEFINE(openat2), i.e. a new `copy_struct_from_user()` is usef that works the same as the kernels version to support backwards-compatibility for struct syscall argument. Instead of including openat2.h we create a copy of `open_how` as `open_how_ver0` to ensure that if the structure grows we can log a LOG_UNIMP warning. Note that in this commit using openat2() for a "faked" file in /proc will ignore the "resolve" flags. This is not great but it seems similar to the exiting behavior when openat() is called with a dirfd to "/proc". Here too the fake file lookup may not catch the special file because "realpath()" is used to determine if the path is in /proc. Alternatively to ignoring we could simply fail with `-TARGET_ENOSYS` (or similar) if `resolve` flags are passed and we found something that looks like a file in /proc that needs faking. Signed-off-by: Michael Vogt <mv...@redhat.com> Buglink: https://github.com/osbuild/bootc-image-builder/issues/619 --- linux-user/syscall.c | 116 ++++++++++++++++++++++++++++++++++++-- linux-user/syscall_defs.h | 7 +++ 2 files changed, 119 insertions(+), 4 deletions(-) diff --git a/linux-user/syscall.c b/linux-user/syscall.c index 9d5415674d..83c944508b 100644 --- a/linux-user/syscall.c +++ b/linux-user/syscall.c @@ -602,6 +602,34 @@ static int check_zeroed_user(abi_long addr, size_t ksize, size_t usize) return 1; } +/* + * Copies a target struct to a host struct, in a way that guarantees + * backwards-compatibility for struct syscall arguments. + * + * Similar to kernels uaccess.h:copy_struct_from_user() + */ +static int +copy_struct_from_user(void *dst, size_t ksize, abi_ptr src, size_t usize) +{ + size_t size = MIN(ksize, usize); + size_t rest = MAX(ksize, usize) - size; + + /* Deal with trailing bytes. */ + if (usize < ksize) { + memset(dst + size, 0, rest); + } else if (usize > ksize) { + int ret = check_zeroed_user(src, ksize, usize); + if (ret <= 0) { + return ret ?: -TARGET_E2BIG; + } + } + /* Copy the interoperable parts of the struct. */ + if (copy_from_user(dst, src, size)) { + return -TARGET_EFAULT; + } + return 0; +} + #define safe_syscall0(type, name) \ static type safe_##name(void) \ { \ @@ -653,6 +681,15 @@ safe_syscall3(ssize_t, read, int, fd, void *, buff, size_t, count) safe_syscall3(ssize_t, write, int, fd, const void *, buff, size_t, count) safe_syscall4(int, openat, int, dirfd, const char *, pathname, \ int, flags, mode_t, mode) + +struct open_how_ver0 { + __u64 flags; + __u64 mode; + __u64 resolve; +}; +safe_syscall4(int, openat2, int, dirfd, const char *, pathname, \ + const struct open_how_ver0 *, how, size_t, size) + #if defined(TARGET_NR_wait4) || defined(TARGET_NR_waitpid) safe_syscall4(pid_t, wait4, pid_t, pid, int *, status, int, options, \ struct rusage *, rusage) @@ -8334,8 +8371,9 @@ static int open_net_route(CPUArchState *cpu_env, int fd) } #endif -int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname, - int flags, mode_t mode, bool safe) +static int maybe_do_fake_open(CPUArchState *cpu_env, int dirfd, + const char *fname, int flags, mode_t mode, + bool safe, bool *use_returned_fd) { g_autofree char *proc_name = NULL; const char *pathname; @@ -8362,6 +8400,7 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname, #endif { NULL, NULL, NULL } }; + *use_returned_fd = true; /* if this is a file from /proc/ filesystem, expand full name */ proc_name = realpath(fname, NULL); @@ -8418,13 +8457,77 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname, return fd; } + *use_returned_fd = false; + return -1; +} + +int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname, + int flags, mode_t mode, bool safe) +{ + bool use_returned_fd; + int fd = maybe_do_fake_open(cpu_env, dirfd, fname, flags, mode, safe, + &use_returned_fd); + if (use_returned_fd) { + return fd; + } + if (safe) { - return safe_openat(dirfd, path(pathname), flags, mode); + return safe_openat(dirfd, path(fname), flags, mode); } else { - return openat(dirfd, path(pathname), flags, mode); + return openat(dirfd, path(fname), flags, mode); } } + +static int do_openat2(CPUArchState *cpu_env, abi_long dirfd, + abi_ptr guest_pathname, abi_ptr guest_open_how, + abi_long guest_size) +{ + struct open_how_ver0 how = {0}; + int ret; + + if (guest_size < sizeof(struct target_open_how_ver0)) { + return -TARGET_EINVAL; + } + ret = copy_struct_from_user(&how, sizeof(how), guest_open_how, guest_size); + if (ret) { + if (ret == -TARGET_E2BIG) { + qemu_log_mask(LOG_UNIMP, + "Unimplemented openat2 open_how size: %lu\n", + guest_size); + } + return ret; + } + char *pathname = lock_user_string(guest_pathname); + if (!pathname) { + return -TARGET_EFAULT; + } + + how.flags = target_to_host_bitmask(how.flags, fcntl_flags_tbl); + how.mode = tswap64(how.mode); + how.resolve = tswap64(how.resolve); + + /* + * Ideally we would pass "how->resolve" flags into this helper too but + * the lookup for files that need faking is based on "realpath()" so + * neither a dirfd for "proc" nor restrictions via "resolve" flags can + * be honored right now. + */ + bool use_returned_fd; + int fd = maybe_do_fake_open(cpu_env, dirfd, pathname, how.flags, how.mode, + true, &use_returned_fd); + if (use_returned_fd) { + return fd; + } else { + ret = get_errno(safe_openat2(dirfd, pathname, &how, + sizeof(struct open_how_ver0))); + } + + fd_trans_unregister(ret); + unlock_user(pathname, guest_pathname, 0); + return ret; +} + ssize_t do_guest_readlink(const char *pathname, char *buf, size_t bufsiz) { ssize_t ret; @@ -9197,6 +9300,11 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1, fd_trans_unregister(ret); unlock_user(p, arg2, 0); return ret; +#if defined(TARGET_NR_openat2) + case TARGET_NR_openat2: + ret = do_openat2(cpu_env, arg1, arg2, arg3, arg4); + return ret; +#endif #if defined(TARGET_NR_name_to_handle_at) && defined(CONFIG_OPEN_BY_HANDLE) case TARGET_NR_name_to_handle_at: ret = do_name_to_handle_at(arg1, arg2, arg3, arg4, arg5); diff --git a/linux-user/syscall_defs.h b/linux-user/syscall_defs.h index a00b617cae..74abcb4613 100644 --- a/linux-user/syscall_defs.h +++ b/linux-user/syscall_defs.h @@ -2754,4 +2754,11 @@ struct target_sched_param { abi_int sched_priority; }; +/* from kernel's include/uapi/linux/openat2.h */ +struct target_open_how_ver0 { + abi_ullong flags; + abi_ullong mode; + abi_ullong resolve; +}; + #endif -- 2.45.2
