From 6c443fb89cfb552fdf76cec9e320b43096f14b18 Mon Sep 17 00:00:00 2001
From: Christian Dietrich <dietrich@ibr.cs.tu-bs.de>
Date: Sun, 24 Dec 2023 00:00:00 +0000
Subject: [PATCH] Dec 24 -- The Endless Advent Calendar
Article: https://ibr.cs.tu-bs.de/advent/24-syscall/
Workload: ~61 source-code lines
---
24-syscall/Makefile | 13 +++++++
24-syscall/usyscall.c | 86 +++++++++++++++++++++++++++++++++++++++++++
2 files changed, 99 insertions(+)
create mode 100644 24-syscall/Makefile
create mode 100644 24-syscall/usyscall.c
diff --git a/24-syscall/Makefile b/24-syscall/Makefile
new file mode 100644
index 0000000..c77a918
--- /dev/null
+++ b/24-syscall/Makefile
@@ -0,0 +1,13 @@
+PROG = usyscall
+
+${PROG}: ${PROG}.c
+ gcc $< -o $@ -Wall -g
+
+run: ${PROG}
+ ./${PROG}
+
+strace: ${PROG}
+ strace ./${PROG}
+
+clean:
+ rm -f ./${PROG}
diff --git a/24-syscall/usyscall.c b/24-syscall/usyscall.c
new file mode 100644
index 0000000..c74e5d8
--- /dev/null
+++ b/24-syscall/usyscall.c
@@ -0,0 +1,86 @@
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <assert.h>
+#include <signal.h>
+#include <sys/ucontext.h>
+#include <sys/uio.h>
+#include <sys/prctl.h>
+#include <sys/syscall.h>
+#include <stdbool.h>
+#include <dlfcn.h>
+#include <ctype.h>
+#include <stdint.h>
+
+
+
+#define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while(0)
+
+// A ROT13 implementation
+#define rot13(c) (isalpha(c)?(c&96)+1+(c-(c&96)+12)%26:c)
+
+// Function prototype
+void usyscall_init(void *offset, ssize_t length);
+void usyscall_signal(int signum, siginfo_t *info, void *context);
+void usyscall_enable(bool enable);
+
+// This flag indicates to the kernel whether system calls are
+// currently blocked or allowed. We start with allowed.
+volatile char usyscall_flag = SYSCALL_DISPATCH_FILTER_ALLOW;
+
+// Enable user space system call dispatching, but exclude the region
+// from [offset, offset+length].
+void usyscall_init(void *offset, ssize_t length) {
+ // FIXME: install SIGSYS signal handler with SA_SIGINFO
+ // FIXME: prctl(2) and PR_SET_SYSCALL_USER_DISPATCH
+}
+
+// Just a wrapper function to enable the usyscall mechanism
+void usyscall_enable(bool enable) {
+ usyscall_flag = enable
+ ? SYSCALL_DISPATCH_FILTER_BLOCK
+ : SYSCALL_DISPATCH_FILTER_ALLOW;
+}
+
+void usyscall_signal(int signum, siginfo_t *info, void *context) {
+ usyscall_enable(false);
+
+ ucontext_t *ctx = (ucontext_t *)context;
+ uint64_t args[6] = {
+ ctx->uc_mcontext.gregs[REG_RDI],
+ ctx->uc_mcontext.gregs[REG_RSI],
+ ctx->uc_mcontext.gregs[REG_RDX],
+ ctx->uc_mcontext.gregs[REG_R10],
+ ctx->uc_mcontext.gregs[REG_R9],
+ ctx->uc_mcontext.gregs[REG_R8]
+ }; (void) args;
+
+ // HINT: Return address can be obtained with:
+ // __builtin_extract_return_addr(__builtin_return_address (0))
+ // FIXME: call usyscall_init(return_address, 20) a second time
+ // FIXME: Interpret some system calls (e.g., __NR_write)
+
+ // A return calls the rt_sigreturn system call. This has to be
+ // allowed here as the (offset+length) of prctl. length=20 bytes
+ // is enough for glibc.
+}
+
+int main(int argc, char **argv) {
+ usyscall_init(NULL, 0);
+
+ write(1, "Hallo Welt\n", 12);
+
+ usyscall_enable(true);
+
+ write(1, "Hallo Welt\n", 12);
+
+ syscall(512, 0xdeadbeef);
+
+ usyscall_enable(false);
+
+ write(1, "Hallo Welt\n", 12);
+
+ return 0;
+}
--
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