grp: Implement fgetgrent_r/getgrent_r

Like fgetpwent_r/getpwent_r, these require a deep copy of the structure
into the caller-provided buffer. This is nontrivial for the array of
strings member.

getgrent_r is required by LSB.

[NOTE: I'm not too happy with the macro, but it works. Any suggestions?]

Signed-off-by: Samuel Holland <samuel@sholland.org>

2 files changed, 99 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 4873403..14c68f4 100644
--- a/Makefile
+++ b/Makefile
@@ -4,6 +4,7 @@ LIBGCOMPAT_SRC = \
 	libgcompat/dlfcn.c		\
 	libgcompat/execinfo.c		\
 	libgcompat/gnulib.c		\
+	libgcompat/grp.c		\
 	libgcompat/malloc.c		\
 	libgcompat/math.c		\
 	libgcompat/pthread.c		\
diff --git a/libgcompat/grp.c b/libgcompat/grp.c
new file mode 100644
index 0000000..5674fda
--- /dev/null
+++ b/libgcompat/grp.c
@@ -0,0 +1,98 @@
+/* some musl versions incorrectly mark fgetgrent() as a GNU extension */
+#define _GNU_SOURCE
+#include <assert.h>  /* assert */
+#include <errno.h>   /* ENOENT, ERANGE */
+#include <grp.h>     /* fgetgrent, getgrent, struct group */
+#include <pthread.h> /* pthread_mutex_* */
+#include <stddef.h>  /* NULL, size_t */
+#include <stdint.h>  /* ptrdiff_t, uintptr_t */
+#include <stdio.h>   /* FILE */
+#include <string.h>  /* memcpy, stpcpy, strlcpy, strlen */
+
+#define ALIGN_PTR_TO_SIZE_OF(ptr, type)                                        \
+	((type *) ((((uintptr_t)(ptr)) + sizeof(type) - 1)                     \
+	           & ~(sizeof(type) - 1)))
+
+static pthread_mutex_t grent_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+static int __fgetgrent_r(FILE *stream, struct group *grp, char *buf, size_t len,
+                         struct group **result)
+{
+	struct group *grtmp;
+	char *cursor = buf, *end = buf + len;
+
+	*result = NULL;
+	pthread_mutex_lock(&grent_mutex);
+	grtmp = stream != NULL ? fgetgrent(stream) : getgrent();
+	if (grtmp == NULL) {
+		pthread_mutex_unlock(&grent_mutex);
+		return ENOENT;
+	}
+	memcpy(grp, grtmp, sizeof(*grp));
+	if (grtmp->gr_name != NULL) {
+		grp->gr_name = cursor;
+		cursor += strlcpy(cursor, grtmp->gr_name, end - cursor) + 1;
+		if (cursor > end) {
+			goto err_unlock;
+		}
+	}
+	if (grtmp->gr_passwd != NULL) {
+		grp->gr_passwd = cursor;
+		cursor += strlcpy(cursor, grtmp->gr_passwd, end - cursor) + 1;
+		if (cursor > end) {
+			goto err_unlock;
+		}
+	}
+	if (grtmp->gr_mem != NULL) {
+		char **members = ALIGN_PTR_TO_SIZE_OF(cursor, char *);
+		ptrdiff_t nameslen = 0;
+		size_t nmem = 0;
+
+		/* Calculate total size of strings plus their pointers. */
+		while (grtmp->gr_mem[nmem++] != NULL) {
+			nameslen += strlen(grtmp->gr_mem[nmem - 1]) + 1;
+		}
+		nameslen += nmem * sizeof(*members);
+		if (nameslen > end - ((char *) members)) {
+			goto err_unlock;
+		}
+		/* Copy the pointers, including the NULL sentinel. */
+		for (size_t i = 0; i < nmem; ++i) {
+			members[i] = grtmp->gr_mem[i];
+		}
+		/* Copy the strings (the NULL sentinel doesn't point to one). */
+		cursor = (char *) &members[nmem];
+		for (size_t i = 0; i < nmem - 1; ++i) {
+			cursor = stpcpy(cursor, members[i]) + 1;
+		}
+	}
+	pthread_mutex_unlock(&grent_mutex);
+	*result = grp;
+
+	return 0;
+
+err_unlock:
+	pthread_mutex_unlock(&grent_mutex);
+	return ERANGE;
+}
+
+/**
+ * Get group file entry.
+ */
+int fgetgrent_r(FILE *stream, struct group *grp, char *buf, size_t len,
+                struct group **result)
+{
+	assert(stream != NULL);
+
+	return __fgetgrent_r(stream, grp, buf, len, result);
+}
+
+/**
+ * Get group database entry.
+ *
+ * LSB 5.0: LSB-Core-generic/baselib-getgrent-r-1.html
+ */
+int getgrent_r(struct group *grp, char *buf, size_t len, struct group **result)
+{
+	return __fgetgrent_r(NULL, grp, buf, len, result);
+}