1 files changed, 170 insertions, 0 deletions
diff --git a/debian/bash.preinst.c b/debian/bash.preinst.c
new file mode 100644
index 0000000..14baeab
--- /dev/null
+++ b/debian/bash.preinst.c
@@ -0,0 +1,170 @@
+/*
+ * This file is in the public domain.
+ * You may freely use, modify, distribute, and relicense it.
+ */
+
+#include "bash.preinst.h"
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+static void backup(const char *file, const char *dest)
+{
+	const char * const cmd[] = {"cp", "-dp", file, dest, NULL};
+	if (exists(file))
+		run(cmd);
+}
+
+static void force_symlink(const char *target, const char *link,
+						const char *temp)
+{
+	/*
+	 * Forcibly create a symlink to "target" from "link".
+	 * This is performed in two stages with an
+	 * intermediate temporary file because symlink(2) cannot
+	 * atomically replace an existing file.
+	 */
+	if ((unlink(temp) && errno != ENOENT) ||
+	    symlink(target, temp) ||
+	    rename(temp, link))
+		die_errno("cannot create symlink %s -> %s", link, target);
+}
+
+static void reset_diversion(const char *package, const char *file,
+						const char *distrib)
+{
+	const char * const remove_old_diversion[] =
+		{"dpkg-divert", "--package", "bash", "--remove", file, NULL};
+	const char * const new_diversion[] =
+		{"dpkg-divert", "--package", package,
+		"--divert", distrib, "--add", file, NULL};
+	run(remove_old_diversion);
+	run(new_diversion);
+}
+
+static int has_binsh_line(FILE *file)
+{
+	char item[sizeof("/bin/sh\n")];
+
+	while (fgets(item, sizeof(item), file)) {
+		int ch;
+
+		if (!memcmp(item, "/bin/sh\n", strlen("/bin/sh\n") + 1))
+			return 1;
+		if (strchr(item, '\n'))
+			continue;
+
+		/* Finish the line. */
+		for (ch = 0; ch != '\n' && ch != EOF; ch = fgetc(file))
+			; /* just reading */
+		if (ch == EOF)
+			break;
+	}
+	if (ferror(file))
+		die_errno("cannot read pipe");
+	return 0;
+}
+
+static int binsh_in_filelist(const char *package)
+{
+	const char * const cmd[] = {"dpkg-query", "-L", package, NULL};
+	pid_t child;
+	int sink;
+	FILE *in;
+	int found;
+
+	/*
+	 * dpkg -L $package 2>/dev/null | ...
+	 *
+	 * Redirection of stderr is for quieter output
+	 * when $package is not installed.  If opening /dev/null
+	 * fails, no problem; leave stderr alone in that case.
+	 */
+	sink = open("/dev/null", O_WRONLY);
+	if (sink >= 0)
+		set_cloexec(sink);
+	in = spawn_pipe(&child, cmd, sink);
+
+	/* ... | grep "^/bin/sh\$" */
+	found = has_binsh_line(in);
+	if (fclose(in))
+		die_errno("cannot close read end of pipe");
+
+	/*
+	 * dpkg -L will error out if $package is not already installed.
+	 *
+	 * We stopped reading early if we found a match, so
+	 * tolerate SIGPIPE in that case.
+	 */
+	wait_or_die(child, "dpkg-query -L", ERROR_OK |
+						(found ? SIGPIPE_OK : 0));
+	return found;
+}
+
+static int undiverted(const char *path)
+{
+	const char * const cmd[] =
+		{"dpkg-divert", "--listpackage", path, NULL};
+	pid_t child;
+	char packagename[sizeof("bash\n")];
+	size_t len;
+	FILE *in = spawn_pipe(&child, cmd, -1);
+	int diverted = 1;
+
+	/* Is $path diverted by someone other than bash? */
+
+	len = fread(packagename, 1, sizeof(packagename), in);
+	if (ferror(in))
+		die_errno("cannot read from dpkg-divert");
+	if (len == 0)
+		diverted = 0;	/* No diversion. */
+	if (len == strlen("bash\n") && !memcmp(packagename, "bash\n", len))
+		diverted = 0;	/* Diverted by bash. */
+
+	if (fclose(in))
+		die_errno("cannot close read end of pipe");
+	wait_or_die(child, "dpkg-divert", ERROR_OK | SIGPIPE_OK);
+	return !diverted;
+}
+
+int main(int argc, char *argv[])
+{
+	/* /bin/sh needs to point to a valid target. */
+
+	if (access("/bin/sh", X_OK)) {
+		backup("/bin/sh", "/bin/sh.distrib");
+		backup("/usr/share/man/man1/sh.1.gz",
+			"/usr/share/man/man1/sh.distrib.1.gz");
+
+		force_symlink("bash", "/bin/sh", "/bin/sh.temp");
+		force_symlink("bash.1.gz", "/usr/share/man/man1/sh.1.gz",
+			"/usr/share/man/man1/sh.1.gz.temp");
+	}
+	if (!binsh_in_filelist("bash"))
+		/* Ready. */
+		return 0;
+
+	/*
+	 * In bash (<= 4.1-3), the bash package included symlinks for
+	 * /bin/sh and the sh(1) manpage in its data.tar.
+	 *
+	 * Unless we are careful, unpacking the new version of bash
+	 * will remove them.  So we tell dpkg that the files from bash
+	 * to be removed are elsewhere, using a diversion on behalf of
+	 * another package.
+	 *
+	 * Based on an idea by Michael Stone.
+	 * “You're one sick individual.” -- Anthony Towns
+	 * http://bugs.debian.org/cgi-bin/bugreport.cgi?msg=85;bug=34717
+	 */
+	if (undiverted("/bin/sh"))
+		reset_diversion("dash", "/bin/sh", "/bin/sh.distrib");
+	if (undiverted("/usr/share/man/man1/sh.1.gz"))
+		reset_diversion("dash", "/usr/share/man/man1/sh.1.gz",
+				"/usr/share/man/man1/sh.distrib.1.gz");
+	return 0;
+}