From dab2c6ee1f308af001dd4f57a13ec16e765f930b Mon Sep 17 00:00:00 2001 From: David Gibson Date: Fri, 24 Jun 2022 12:17:29 +1000 Subject: Add cleaner line-by-line reading primitives Two places in passt need to read files line by line (one parsing resolv.conf, the other parsing /proc/net/*. They can't use fgets() because in glibc that can allocate memory. Instead they use an implementation line_read() in util.c. This has some problems: * It has two completely separate modes of operation, one buffering and one not, the relation between these and how they're activated is subtle and confusing * At least in non-buffered mode, it will mishandle an empty line, folding them onto the start of the next non-empty line * In non-buffered mode it will use lseek() which prevents using this on non-regular files (we don't need that at present, but it's a surprising limitation) * It has a lot of difficult to read pointer mangling Add a new cleaner implementation of allocation-free line-by-line reading in lineread.c. This one always buffers, using a state structure to keep track of what we need. This is larger than I'd like, but it turns out handling all the edge cases of line-by-line reading in C is surprisingly hard. This just adds the code, subsequent patches will change the existing users of line_read() to the new implementation. Signed-off-by: David Gibson --- lineread.c | 115 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 115 insertions(+) create mode 100644 lineread.c (limited to 'lineread.c') diff --git a/lineread.c b/lineread.c new file mode 100644 index 0000000..59021e6 --- /dev/null +++ b/lineread.c @@ -0,0 +1,115 @@ +// SPDX-License-Identifier: AGPL-3.0-or-later + +/* PASST - Plug A Simple Socket Transport + * for qemu/UNIX domain socket mode + * + * PASTA - Pack A Subtle Tap Abstraction + * for network namespace/tap device mode + * + * lineread.c - Allocation free line-by-line buffered file input + * + * Copyright Red Hat + * Author: David Gibson + */ + +#include +#include +#include +#include +#include +#include + +#include "lineread.h" + +/** + * lineread_init() - Prepare for line by line file reading without allocation + * @lr: Line reader state structure to initialize + * @fd: File descriptor to read lines from + */ +void lineread_init(struct lineread *lr, int fd) +{ + lr->fd = fd; + lr->next_line = lr->count = 0; +} + +/** + * peek_line() - Find and NULL-terminate next line in buffer + * @lr: Line reader state structure + * @eof: Caller indicates end-of-file was already found by read() + * + * Return: length of line in bytes, -1 if no line was found + */ +static int peek_line(struct lineread *lr, bool eof) +{ + char *nl; + + /* Sanity checks (which also document invariants) */ + assert(lr->count >= 0); + assert(lr->next_line >= 0); + assert(lr->next_line + lr->count >= lr->next_line); + assert(lr->next_line + lr->count <= LINEREAD_BUFFER_SIZE); + + nl = memchr(lr->buf + lr->next_line, '\n', lr->count); + + if (nl) { + *nl = '\0'; + return nl - lr->buf - lr->next_line + 1; + } + + if (eof) { + lr->buf[lr->next_line + lr->count] = '\0'; + /* No trailing newline, so treat all remaining bytes + * as the last line + */ + return lr->count; + } + + return -1; +} + +/** + * lineread_get() - Read a single line from file (no allocation) + * @lr: Line reader state structure + * @line: Place a pointer to the next line in this variable + * + * Return: Length of line read on success, 0 on EOF, negative on error + */ +int lineread_get(struct lineread *lr, char **line) +{ + bool eof = false; + int line_len; + + while ((line_len = peek_line(lr, eof)) < 0) { + int rc; + + if ((lr->next_line + lr->count) == LINEREAD_BUFFER_SIZE) { + /* No space at end */ + if (lr->next_line == 0) { + /* Buffer is full, which means we've + * hit a line too long for us to + * process. FIXME: report error + * better + */ + return -1; + } + memmove(lr->buf, lr->buf + lr->next_line, lr->count); + lr->next_line = 0; + } + + /* Read more data into the end of buffer */ + rc = read(lr->fd, lr->buf + lr->next_line + lr->count, + LINEREAD_BUFFER_SIZE - lr->next_line - lr->count); + if (rc < 0) + return rc; + + if (rc == 0) + eof = true; + else + lr->count += rc; + } + + *line = lr->buf + lr->next_line; + lr->next_line += line_len; + lr->count -= line_len; + return line_len; +} -- cgit v1.2.3