autodata: stash pointers in a binary.

This is a more portable variant of the ELF section trick.

ccan/autodata/LICENSE

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+../../licenses/BSD-MIT

ccan/autodata/_info

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+#include <string.h>
+#include "config.h"
+
+/**
+ * autodata - stash pointers in your binary for automatic registration
+ *
+ * This code allows declarations in your source which you can gather
+ * together at runtime to form tables.  This is often used in place of
+ * having to patch a registration function call out, or a central
+ * table.
+ *
+ * License: BSD-MIT
+ */
+int main(int argc, char *argv[])
+{
+	/* Expect exactly one argument */
+	if (argc != 2)
+		return 1;
+
+	if (strcmp(argv[1], "depends") == 0) {
+		printf("ccan/compiler\n");
+		printf("ccan/ptr_valid\n");
+		return 0;
+	}
+
+	return 1;
+}

ccan/autodata/autodata.c

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+// Licensed under BSD-MIT: See LICENSE.
+#include "autodata.h"
+#include <stdint.h>
+#include <unistd.h>
+#include <string.h>
+
+#if HAVE_SECTION_START_STOP
+void *autodata_get_section(void *start, void *stop, size_t *nump)
+{
+	*nump = (void **)(stop) - (void **)(start);
+	return start;
+}
+
+void autodata_free(void *table)
+{
+}
+#else
+#include <ccan/ptr_valid/ptr_valid.h>
+
+void *autodata_make_table(const void *example, const char *name, size_t *nump)
+{
+	const char *start, *end, *tag;
+	struct ptr_valid_batch batch;
+	const void *const magic = (void *)AUTODATA_MAGIC;
+	void **table = NULL;
+	char first_magic;
+
+	if (!ptr_valid_batch_start(&batch))
+		return NULL;
+
+	/* Get range to search. */
+	for (start = (char *)((intptr_t)example & ~(getpagesize() - 1));
+	     ptr_valid_batch(&batch, start-getpagesize(), 1, sizeof(void *),
+			     false);
+	     start -= getpagesize());
+
+	for (end = (char *)((intptr_t)example & ~(getpagesize() - 1));
+	     ptr_valid_batch(&batch, end, 1, sizeof(void *), false);
+	     end += getpagesize());
+
+	*nump = 0;
+	first_magic = *(char *)&magic;
+	for (tag = memchr(start, first_magic, end - start);
+	     tag;
+	     tag = memchr(tag+1, first_magic, end - (tag + 1))) {
+		void *adata[4];
+
+		/* We can read 4 void *'s here? */
+		if (tag + sizeof(adata) > end)
+			continue;
+
+		memcpy(adata, tag, sizeof(adata));
+
+		/* False match? */
+		if (adata[0] != (void *)AUTODATA_MAGIC || adata[1] != tag)
+			continue;
+
+		/* OK, check name. */
+		if (!ptr_valid_batch_string(&batch, adata[3])
+		    || strcmp(name, adata[3]) != 0)
+			continue;
+
+		if (!ptr_valid_batch_read(&batch, (char *)adata[2]))
+			continue;
+
+		table = realloc(table, sizeof(void *) * (*nump + 1));
+		if (!table)
+			break;
+		table[*nump] = adata[2];
+		(*nump)++;
+	}
+	ptr_valid_batch_end(&batch);
+	return table;
+}
+
+void autodata_free(void *table)
+{
+	free(table);
+}
+#endif

ccan/autodata/autodata.h

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+// Licensed under BSD-MIT: See LICENSE.
+#ifndef CCAN_AUTODATA_H
+#define CCAN_AUTODATA_H
+#include "config.h"
+#include <ccan/compiler/compiler.h>
+#include <stdlib.h>
+
+#if HAVE_SECTION_START_STOP
+
+/**
+ * AUTODATA_TYPE - declare the type for a given autodata name.
+ * @name: the name for this set of autodata
+ * @type: the type this autodata points to
+ *
+ * This macro is usually placed in a header: it must preceed any
+ * autodata functions in the file.
+ *
+ * Example:
+ *	// My set of char pointers.
+ *	AUTODATA_TYPE(names, char);
+ */
+#define AUTODATA_TYPE(name, type)					\
+	typedef type autodata_##name##_;				\
+	extern type *__start_autodata_##name[], *__stop_autodata_##name[]
+
+/**
+ * AUTODATA - add a pointer to this autodata set
+ * @name: the name of the set of autodata
+ * @ptr: the compile-time-known pointer
+ *
+ * This embeds @ptr into the binary, with the tag corresponding to
+ * @name (which must look like a valid identifier, no punctuation!).
+ * The type of @ptr must match that given by AUTODATA_TYPE.  It is
+ * usually a file-level declaration.
+ *
+ * Example:
+ *	// Put two char pointers into the names AUTODATA set.
+ *	AUTODATA(names, "Arabella");
+ *	AUTODATA(names, "Alex");
+ */
+#define AUTODATA(name, ptr) \
+	static const autodata_##name##_ *NEEDED		\
+	__attribute__((section("autodata_" #name)))	\
+	AUTODATA_VAR_(name, __LINE__) = (ptr);
+
+/**
+ * autodata_get - get an autodata set
+ * @name: the name of the set of autodata
+ * @nump: the number of items in the set.
+ *
+ * This extract the embedded pointers matching @name.  It may fail
+ * if malloc() fails, or if there is no AUTODATA at all.
+ *
+ * The return will be a pointer to an array of @type pointers (from
+ * AUTODATA_TYPE).
+ *
+ * Example:
+ *	static void print_embedded_names(void)
+ *	{
+ *		unsigned int i, num;
+ *		char **n = autodata_get(names, &num);
+ *
+ *		for (i = 0; i < num; i++)
+ *			printf("%s\n", n[i]);
+ *	}
+ */
+#define autodata_get(name, nump)					\
+	((autodata_##name##_ **)					\
+	 autodata_get_section(__start_autodata_##name,			\
+			      __stop_autodata_##name, (nump)))
+#endif /* HAVE_SECTION_START_STOP */
+
+/**
+ * autodata_free - free the table returned by autodata_get()
+ * @p: the table.
+ */
+void autodata_free(void *p);
+
+/* Internal functions. */
+#define AUTODATA_VAR__(name, line) autodata_##name##_##line
+#define AUTODATA_VAR_(name, line) AUTODATA_VAR__(name, line)
+
+#if HAVE_SECTION_START_STOP
+void *autodata_get_section(void *start, void *stop, size_t *nump);
+#else
+#define AUTODATA_TYPE(name, type)					\
+	typedef type autodata_##name##_;				\
+	static const void *autodata_##name##_ex = &autodata_##name##_ex
+
+#define AUTODATA_MAGIC ((long)0xFEEDA10DA7AF00D5ULL)
+#define AUTODATA(name, ptr)						\
+	static const autodata_##name##_ *NEEDED				\
+	AUTODATA_VAR_(name, __LINE__)[4] =				\
+	{ (void *)AUTODATA_MAGIC,					\
+	  (void *)&AUTODATA_VAR_(name, __LINE__),			\
+	  (ptr),							\
+	  (void *)#name }
+
+#define autodata_get(name, nump)					\
+	((autodata_##name##_ **)					\
+	 autodata_make_table(&autodata_##name##_ex, #name, (nump)))
+
+void *autodata_make_table(const void *example, const char *name, size_t *nump);
+#endif
+
+#endif /* CCAN_AUTODATA_H */

ccan/autodata/test/helper.c

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+/* Check that linking together works. */
+#include <ccan/autodata/autodata.h>
+
+AUTODATA_TYPE(autostrings, char);
+
+AUTODATA(autostrings, "helper");

ccan/autodata/test/run-fools.c

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+#include <ccan/autodata/autodata.h>
+/* Include the C files directly. */
+#include <ccan/autodata/autodata.c>
+#include <ccan/tap/tap.h>
+
+AUTODATA_TYPE(autostrings, char);
+
+AUTODATA(autostrings, "genuine");
+
+#if !HAVE_SECTION_START_STOP
+/* These are all fake, to test the various failure paths. */
+/* Hopefully fake_alpha or fake_omega will test run-past-end. */
+static const void *NEEDED fake_alpha[] = { (void *)AUTODATA_MAGIC };
+
+/* Wrong magic in the middle. */
+static const void *NEEDED fake1[] = { (void *)(AUTODATA_MAGIC ^ 0x10000),
+				      (void *)&fake1,
+				      "fake1",
+				      (void *)"autostrings" };
+
+/* Wrong self pointer. */
+static const void *NEEDED fake2[] = { (void *)AUTODATA_MAGIC,
+				      (void *)&fake1,
+				      "fake2",
+				      (void *)"autostrings" };
+
+/* Wrong name. */
+static const void *NEEDED fake3[] = { (void *)AUTODATA_MAGIC,
+				      (void *)&fake3,
+				      "fake3",
+				      (void *)"autostrings2" };
+
+/* Invalid self-pointer. */
+static const void *NEEDED fake4[] = { (void *)AUTODATA_MAGIC,
+				      (void *)1UL,
+				      "fake4",
+				      (void *)"autostrings" };
+
+/* Invalid name pointer */
+static const void *NEEDED fake5[] = { (void *)AUTODATA_MAGIC,
+				      (void *)&fake5,
+				      "fake5",
+				      (void *)1UL };
+
+/* Invalid contents pointer */
+static const void *NEEDED fake6[] = { (void *)AUTODATA_MAGIC,
+				      (void *)&fake6,
+				      (char *)1UL,
+				      (void *)"autostrings" };
+
+static const void *NEEDED fake_omega[] = { (void *)AUTODATA_MAGIC };
+#endif
+
+int main(void)
+{
+	char **table;
+	size_t num;
+
+	/* This is how many tests you plan to run */
+	plan_tests(2);
+
+	table = autodata_get(autostrings, &num);
+	ok1(num == 2);
+	ok1((!strcmp(table[0], "genuine") && !strcmp(table[1], "helper"))
+	    || (!strcmp(table[1], "genuine") && !strcmp(table[0], "helper")));
+
+	autodata_free(table);
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}

ccan/autodata/test/run.c

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+#include <ccan/autodata/autodata.h>
+/* Include the C files directly. */
+#include <ccan/autodata/autodata.c>
+#include <ccan/tap/tap.h>
+
+AUTODATA_TYPE(autostrings, char);
+
+AUTODATA(autostrings, "hello");
+AUTODATA(autostrings, "world");
+
+int main(void)
+{
+	char **table;
+	size_t num;
+	int i, hello = -1, world = -1, helper = -1;
+
+	/* This is how many tests you plan to run */
+	plan_tests(4);
+
+	table = autodata_get(autostrings, &num);
+	ok1(num == 3);
+
+	for (i = 0; i < num; i++) {
+		if (strcmp(table[i], "hello") == 0)
+			hello = i;
+		else if (strcmp(table[i], "world") == 0)
+			world = i;
+		else if (strcmp(table[i], "helper") == 0)
+			helper = i;
+		else
+			fail("Unknown entry %s", table[i]);
+	}
+	ok1(hello != -1);
+	ok1(world != -1);
+	ok1(helper != -1);
+
+	autodata_free(table);
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}