zvfs: improve libc FILE to integer fd abstraction

Previously, there was an implicit assumption that Zephyr's
internal struct fd_entry * was synonymous with FILE * from
the C library.

This is generally not the case and aliasing these two
distinct types was preventing a fair bit of functionality
from Just Working - namely stdio function calls like
fgets() and fopen(). The problem count be seen directly
when trying to use a function like zvfs_fdopen().

Instead of aliasing the two types, require that all Zephyr
C libraries provide

1. FILE *z_libc_file_alloc(int fd, const char *mode)
   Allocate and populate the required fields of a FILE object

2. int z_libc_file_get_fd(FILE *fp)
   Convert a FILE* object to an integer file descriptor.

For Picolibc and Newlib-based C libraries, these functions
set and get the integer file descriptor from a field of the
internal FILE object representation.

For the minimal C library, these functions convert between
array index and struct fd_entry pointers.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

Author: cfriedt

lib/libc/newlib/CMakeLists.txt

@@ -1,7 +1,10 @@
 # SPDX-License-Identifier: Apache-2.0
 
 zephyr_library()
-zephyr_library_sources(libc-hooks.c)
+zephyr_library_sources(
+  libc-hooks.c
+  z_libc.c
+)
 
 # Do not allow LTO when compiling libc-hooks.c file
 set_source_files_properties(libc-hooks.c PROPERTIES COMPILE_OPTIONS $<TARGET_PROPERTY:compiler,prohibit_lto>)

lib/libc/newlib/z_libc.c

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2024 Tenstorrent AI ULC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <zephyr/sys/fdtable.h>
+
+static int z_libc_sflags(const char *mode)
+{
+	int ret = 0;
+
+	switch (mode[0]) {
+	case 'r':
+		ret = ZVFS_O_RDONLY;
+		break;
+
+	case 'w':
+		ret = ZVFS_O_WRONLY | ZVFS_O_CREAT | ZVFS_O_TRUNC;
+		break;
+
+	case 'a':
+		ret = ZVFS_O_WRONLY | ZVFS_O_CREAT | ZVFS_O_APPEND;
+		break;
+	default:
+		return 0;
+	}
+
+	while (*++mode) {
+		switch (*mode) {
+		case '+':
+			ret |= ZVFS_O_RDWR;
+			break;
+		case 'x':
+			ret |= ZVFS_O_EXCL;
+			break;
+		default:
+			break;
+		}
+	}
+
+	return ret;
+}
+
+FILE *z_libc_file_alloc(int fd, const char *mode)
+{
+	FILE *fp;
+	/*
+	 * These symbols have conflicting declarations in upstream headers.
+	 * Use uintptr_t and a cast to assign cleanly.
+	 */
+	extern uintptr_t _close_r;
+	extern uintptr_t _lseek_r;
+	extern uintptr_t _read_r;
+	extern uintptr_t _write_r;
+
+	fp = calloc(1, sizeof(*fp));
+	if (fp == NULL) {
+		return NULL;
+	}
+
+	fp->_flags = z_libc_sflags(mode);
+	fp->_file = fd;
+	fp->_cookie = (void *)fp;
+
+	*(uintptr_t *)fp->_read = _read_r;
+	*(uintptr_t *)fp->_write = _write_r;
+	*(uintptr_t *)fp->_seek = _lseek_r;
+	*(uintptr_t *)fp->_close = _close_r;
+
+	return fp;
+}
+
+int z_libc_file_get_fd(FILE *fp)
+{
+	return fp->_file;
+}

lib/libc/picolibc/CMakeLists.txt

@@ -9,6 +9,7 @@ zephyr_library_sources(
   exit.c
   locks.c
   stdio.c
+  z_libc.c
   )
 
 zephyr_library_compile_options(-fno-lto)

lib/libc/picolibc/z_libc.c

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2024 Tenstorrent AI ULC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "stdio-bufio.h"
+
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <zephyr/sys/fdtable.h>
+
+#define FDEV_SETUP_ZVFS(fd, buf, size, rwflags, bflags)                                            \
+	FDEV_SETUP_BUFIO(fd, buf, size, zvfs_read_wrap, zvfs_write_wrap, zvfs_lseek, zvfs_close,   \
+			 rwflags, bflags)
+
+int __posix_sflags(const char *mode, int *optr);
+
+/* FIXME: do not use ssize_t or off_t */
+ssize_t zvfs_read(int fd, void *buf, size_t sz, const size_t *from_offset);
+ssize_t zvfs_write(int fd, const void *buf, size_t sz, const size_t *from_offset);
+off_t zvfs_lseek(int fd, off_t offset, int whence);
+int zvfs_close(int fd);
+
+static ssize_t zvfs_read_wrap(int fd, void *buf, size_t sz)
+{
+	return zvfs_read(fd, buf, sz, NULL);
+}
+
+static ssize_t zvfs_write_wrap(int fd, const void *buf, size_t sz)
+{
+	return zvfs_write(fd, buf, sz, NULL);
+}
+
+FILE *z_libc_file_alloc(int fd, const char *mode)
+{
+	struct __file_bufio *bf;
+	int __maybe_unused unused;
+
+	bf = calloc(1, sizeof(struct __file_bufio) + BUFSIZ);
+	if (bf == NULL) {
+		return NULL;
+	}
+
+	*bf = (struct __file_bufio)FDEV_SETUP_ZVFS(fd, (char *)(bf + 1), BUFSIZ,
+						   __posix_sflags(mode, &unused), __BFALL);
+
+	__bufio_lock_init(&(bf->xfile.cfile.file));
+
+	return &(bf->xfile.cfile.file);
+}
+
+int z_libc_file_get_fd(const FILE *fp)
+{
+	const struct __file_bufio *const bf = (const struct __file_bufio *)fp;
+
+	return POINTER_TO_INT(bf->ptr);
+}

lib/os/fdtable.c

@@ -24,6 +24,11 @@
 #include <zephyr/internal/syscall_handler.h>
 #include <zephyr/sys/atomic.h>
 
+#ifndef CONFIG_MINIMAL_LIBC
+extern FILE *z_libc_file_alloc(int fd, const char *mode);
+extern int z_libc_file_get_fd(const FILE *fp);
+#endif
+
 struct stat;
 
 struct fd_entry {
@@ -75,6 +80,20 @@ static struct fd_entry fdtable[CONFIG_ZVFS_OPEN_MAX] = {
 
 static K_MUTEX_DEFINE(fdtable_lock);
 
+#ifdef CONFIG_MINIMAL_LIBC
+static ALWAYS_INLINE inline FILE *z_libc_file_alloc(int fd, const char *mode)
+{
+	ARG_UNUSED(mode);
+
+	return (FILE *)&fdtable[fd];
+}
+
+static ALWAYS_INLINE inline int z_libc_file_get_fd(const FILE *fp)
+{
+	return (const struct fd_entry *)fp - fdtable;
+}
+#endif
+
 static int z_fd_ref(int fd)
 {
 	return atomic_inc(&fdtable[fd].refcount) + 1;
@@ -413,23 +432,30 @@ int zvfs_close(int fd)
 
 FILE *zvfs_fdopen(int fd, const char *mode)
 {
-	ARG_UNUSED(mode);
+	FILE *ret;
 
 	if (_check_fd(fd) < 0) {
 		return NULL;
 	}
 
-	return (FILE *)&fdtable[fd];
+	ret = z_libc_file_alloc(fd, mode);
+	if (ret == NULL) {
+		errno = ENOMEM;
+	}
+
+	return ret;
 }
 
 int zvfs_fileno(FILE *file)
 {
-	if (!IS_ARRAY_ELEMENT(fdtable, file)) {
+	int fd = z_libc_file_get_fd(file);
+
+	if (fd < 0 || fd >= ARRAY_SIZE(fdtable)) {
 		errno = EBADF;
 		return -1;
 	}
 
-	return (struct fd_entry *)file - fdtable;
+	return fd;
 }
 
 int zvfs_fstat(int fd, struct stat *buf)