realpath: Intercept realpath(/proc/self/exe)

Reuse the machinery from readlink

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

3 files changed, 55 insertions, 2 deletions

diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 2a4b1d3..188d75c 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -35,9 +35,10 @@ math
 * Add most __*_finite functions, courtesy of Elizabeth Myers.
 
 
-readlink
---------
+readlink/realpath
+-----------------
 
+* Intercept realpath as well as readlink.
 * Report ENOSYS on dlsym failure.
 
 
diff --git a/Makefile b/Makefile
index 272bfd9..934422f 100644
--- a/Makefile
+++ b/Makefile
@@ -16,6 +16,7 @@ LIBGCOMPAT_SRC = \
 	libgcompat/pthread.c		\
 	libgcompat/pwd.c		\
 	libgcompat/readlink.c		\
+	libgcompat/realpath.c		\
 	libgcompat/resolv.c		\
 	libgcompat/resource.c		\
 	libgcompat/setjmp.c		\
diff --git a/libgcompat/realpath.c b/libgcompat/realpath.c
new file mode 100644
index 0000000..ab68cc4
--- /dev/null
+++ b/libgcompat/realpath.c
@@ -0,0 +1,51 @@
+#include <dlfcn.h>
+#include <errno.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 *(*real_realpath)(const char *restrict, char *restrict);
+
+char *realpath(const char *restrict path, char *restrict resolved)
+{
+	if (real_realpath == NULL) {
+		real_realpath = dlsym(RTLD_NEXT, "realpath");
+		if (real_realpath == NULL) {
+			errno = ENOSYS;
+			return NULL;
+		}
+	}
+
+	if (!strcmp(path, "/proc/self/exe")) {
+		char *fixed = resolved;
+
+		if (fixed == NULL) {
+			fixed = malloc(PATH_MAX);
+			if (fixed == NULL) {
+				errno = ENOMEM;
+				return NULL;
+			}
+		}
+
+		/* If passed in, the buffer is at least PATH_MAX per POSIX. */
+		ssize_t written = readlink(path, fixed, PATH_MAX - 1);
+		if (written == -1) {
+			/* Free the buffer iff we allocated it. */
+			if (fixed != resolved)
+				free(fixed);
+			return NULL;
+		}
+		fixed[written] = '\0';
+
+		return fixed;
+	}
+
+	return real_realpath(path, resolved);
+}