readlink: Intercept readlink("/proc/self/exe")

This allows programs run through gcompat to fork and re-exec themselves.
It fixes readlink("/proc/self/exe") to return the executable's absolute
path, instead of musl's path.

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

2 files changed, 86 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 5e5a12e..b6bee29 100644
--- a/Makefile
+++ b/Makefile
@@ -12,6 +12,7 @@ LIBGCOMPAT_SRC = \
 	libgcompat/netdb.c		\
 	libgcompat/pthread.c		\
 	libgcompat/pwd.c		\
+	libgcompat/readlink.c		\
 	libgcompat/resolv.c		\
 	libgcompat/resource.c		\
 	libgcompat/setjmp.c		\
diff --git a/libgcompat/readlink.c b/libgcompat/readlink.c
new file mode 100644
index 0000000..63def96
--- /dev/null
+++ b/libgcompat/readlink.c
@@ -0,0 +1,85 @@
+#include <dlfcn.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#ifndef LINKER
+#error LINKER must be defined
+#endif
+
+static char exe[PATH_MAX], *linker;
+static ssize_t (*real_readlink)(const char *, char *, size_t);
+
+ssize_t readlink(const char *path, char *buf, size_t len)
+{
+	if (real_readlink == NULL) {
+		real_readlink = dlsym(RTLD_NEXT, "readlink");
+		if (real_readlink == NULL) {
+			return -1;
+		}
+	}
+
+	if (!strcmp(path, "/proc/self/exe")) {
+		int fd;
+
+		if (exe[0] == '\0') {
+			if (linker == NULL) {
+				linker = realpath(LINKER, NULL);
+				if (linker == NULL) {
+					return -1;
+				}
+			}
+			if (real_readlink(path, exe, sizeof(exe)) < 1) {
+				goto fail;
+			}
+			if (!strcmp(exe, linker)) {
+				char c;
+				int arg = 0;
+				ssize_t arglen;
+
+				fd = open("/proc/self/cmdline",
+				          O_RDONLY | O_CLOEXEC);
+				if (fd < 0) {
+					goto fail;
+				}
+				/* Skip the --argv0/--preload ldso args.
+				 * This number must be kept in sync with the
+				 * argument order in loader/loader.c */
+				while (arg < 5) {
+					if (read(fd, &c, 1) != 1) {
+						goto fail_close;
+					}
+					if (c == '\0') {
+						++arg;
+					}
+				}
+				/* Read the executable path from the cmdline. */
+				arglen = read(fd, exe, sizeof(exe));
+				if (arglen < 1) {
+					goto fail_close;
+				}
+				close(fd);
+				/* Ensure the path exists, fits, and has NUL. */
+				if (exe[0] == '\0') {
+					goto fail;
+				}
+				if (strnlen(exe, arglen) == (size_t) arglen) {
+					goto fail;
+				}
+			}
+		}
+
+		return stpncpy(buf, exe, len) - buf;
+
+	fail_close:
+		close(fd);
+	fail:
+		exe[0] = '\0';
+		return -1;
+	}
+
+	return real_readlink(path, buf, len);
+}