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Date: Thu, 12 Oct 2017 00:05:20 +0000
From: William Pitcock <nenolod@...eferenced.org>
To: musl@...ts.openwall.com
Cc: William Pitcock <nenolod@...eferenced.org>
Subject: [PATCH v6] stdio: implement fopencookie(3)

The fopencookie(3) function allows the programmer to create a custom
stdio implementation, using four hook functions which operate on a
"cookie" data type.

Changelog:

v6:
- remove pointer arithmetic instead using a structure to contain the parent FILE
  object
- set F_ERR flag where appropriate
- style fixes
- fix stdio readahead to handle single-byte read case (as pointed out by dalias,
  tested by custom fuzzer)
- handle seek return values correctly (found by fuzzing)

v5:
- implement stdio readahead buffering support

v4:
- remove parameter names from header function declarations

v3:
- remove spurious `struct winsize`
- make f->lock unconditionally 0

v2:
- properly implement stdio buffering

v1:
- initial proof of concept
---
 include/stdio.h         |   9 ++++
 src/stdio/fopencookie.c | 141 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 150 insertions(+)
 create mode 100644 src/stdio/fopencookie.c

diff --git a/include/stdio.h b/include/stdio.h
index 884d2e6a..da0563f6 100644
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -182,6 +182,15 @@ int vasprintf(char **, const char *, __isoc_va_list);
 #ifdef _GNU_SOURCE
 char *fgets_unlocked(char *, int, FILE *);
 int fputs_unlocked(const char *, FILE *);
+
+typedef struct {
+	ssize_t (*read)(void *, char *, size_t);
+	ssize_t (*write)(void *, const char *, size_t);
+	int (*seek)(void *, off_t *, int);
+	int (*close)(void *);
+} cookie_io_functions_t;
+
+FILE *fopencookie(void *, const char *, cookie_io_functions_t);
 #endif
 
 #if defined(_LARGEFILE64_SOURCE) || defined(_GNU_SOURCE)
diff --git a/src/stdio/fopencookie.c b/src/stdio/fopencookie.c
new file mode 100644
index 00000000..bcf42c10
--- /dev/null
+++ b/src/stdio/fopencookie.c
@@ -0,0 +1,141 @@
+#define _GNU_SOURCE
+#include "stdio_impl.h"
+#include <stdlib.h>
+#include <sys/ioctl.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+
+struct fcookie {
+	void *cookie;
+	cookie_io_functions_t iofuncs;
+};
+
+struct cookie_FILE {
+	FILE f;
+	struct fcookie fc;
+	unsigned char buf[UNGET+BUFSIZ];
+};
+
+static size_t cookieread(FILE *f, unsigned char *buf, size_t len)
+{
+	struct fcookie *fc = f->cookie;
+	ssize_t ret = -1;
+	size_t remain = len, readlen = 0;
+
+	if (!fc->iofuncs.read) goto bail;
+
+	ret = fc->iofuncs.read(fc->cookie, (char *) buf, len > 1 ? (len - 1) : 1);
+	if (ret <= 0) goto bail;
+
+	readlen += ret;
+	remain -= ret;
+
+	f->rpos = f->buf;
+	ret = fc->iofuncs.read(fc->cookie, (char *) f->rpos, f->buf_size);
+	if (ret <= 0) goto bail;
+	f->rend = f->rpos + ret;
+
+	if (remain > 0) {
+		if (remain > f->buf_size) remain = f->buf_size;
+		memcpy(buf + readlen, f->rpos, remain);
+		readlen += remain;
+		f->rpos += remain;
+	}
+
+	return readlen;
+
+bail:
+	f->flags |= F_EOF ^ ((F_ERR^F_EOF) & ret);
+	f->rpos = f->rend = f->buf;
+	return readlen;
+}
+
+static size_t cookiewrite(FILE *f, const unsigned char *buf, size_t len)
+{
+	struct fcookie *fc = f->cookie;
+	ssize_t ret;
+	size_t len2 = f->wpos - f->wbase;
+	if (!fc->iofuncs.write) return len;
+	if (len2) {
+		f->wpos = f->wbase;
+		if (cookiewrite(f, f->wpos, len2) < len2) return 0;
+	}
+	ret = fc->iofuncs.write(fc->cookie, (const char *) buf, len);
+	if (ret < 0) {
+		f->wpos = f->wbase = f->wend = 0;
+		f->flags |= F_ERR;
+		return 0;
+	}
+	return ret;
+}
+
+static off_t cookieseek(FILE *f, off_t off, int whence)
+{
+	struct fcookie *fc = f->cookie;
+	int res;
+	if (whence > 2) {
+		errno = EINVAL;
+		return -1;
+	}
+	if (!fc->iofuncs.seek) {
+		errno = ENOTSUP;
+		return -1;
+	}
+	res = fc->iofuncs.seek(fc->cookie, &off, whence);
+	if (res < 0)
+		return res;
+	return off;
+}
+
+static int cookieclose(FILE *f)
+{
+	struct fcookie *fc = f->cookie;
+	if (fc->iofuncs.close) return fc->iofuncs.close(fc->cookie);
+	return 0;
+}
+
+FILE *fopencookie(void *cookie, const char *mode, cookie_io_functions_t iofuncs)
+{
+	struct cookie_FILE *f;
+
+	/* Check for valid initial mode character */
+	if (!strchr("rwa", *mode)) {
+		errno = EINVAL;
+		return 0;
+	}
+
+	/* Allocate FILE+fcookie+buffer or fail */
+	if (!(f=malloc(sizeof *f))) return 0;
+
+	/* Zero-fill only the struct, not the buffer */
+	memset(f, 0, sizeof(FILE));
+
+	/* Impose mode restrictions */
+	if (!strchr(mode, '+')) f->f.flags = (*mode == 'r') ? F_NOWR : F_NORD;
+
+	/* Set up our fcookie */
+	f->fc.cookie = cookie;
+	f->fc.iofuncs.read = iofuncs.read;
+	f->fc.iofuncs.write = iofuncs.write;
+	f->fc.iofuncs.seek = iofuncs.seek;
+	f->fc.iofuncs.close = iofuncs.close;
+
+	f->f.fd = -1;
+	f->f.cookie = &f->fc;
+	f->f.buf = f->buf;
+	f->f.buf_size = BUFSIZ;
+	f->f.lbf = EOF;
+
+	/* enable opportunistic stdio locking */
+	f->f.lock = 0;
+
+	/* Initialize op ptrs. No problem if some are unneeded. */
+	f->f.read = cookieread;
+	f->f.write = cookiewrite;
+	f->f.seek = cookieseek;
+	f->f.close = cookieclose;
+
+	/* Add new FILE to open file list */
+	return __ofl_add(&f->f);
+}
-- 
2.13.3

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