factorize printf into io.h

3 files changed, 49 insertions, 20 deletions

diff --git a/02_process_init/Makefile b/02_process_init/Makefile
index 6e1f512..430b0a9 100644
--- a/02_process_init/Makefile
+++ b/02_process_init/Makefile
@@ -1,18 +1,16 @@
 # Copyright (c) 2020 Johannes Stoelp
 
 show: entry
-	#gdb -q --batch -ex 'starti' -ex 'x/g $$rsp' -ex 'x/s *(char**)($$rsp+8)' ./$^
 	./entry 1 2 3 4
 
 entry: entry.S entry.c
 	gcc -o $@                        \
-	    -I ../include                \
+	    -g -O0                       \
 	    -Wall -Wextra                \
-	    -static                      \
+	    -I ../include                \
 	    -nostartfiles -nodefaultlibs \
-	    -g -O0                       \
+	    -static                      \
 	    $^
 
-
 clean:
 	rm -f entry
diff --git a/02_process_init/entry.c b/02_process_init/entry.c
index a6b0918..360d205 100644
--- a/02_process_init/entry.c
+++ b/02_process_init/entry.c
@@ -1,26 +1,14 @@
 // Copyright (c) 2020 Johannes Stoelp
 
-#include <asm/unistd.h>
 #include <elf.h>
-#include <fmt.h>
-#include <stdint.h>
-#include <syscall.h>
+#define MAX_PRINTF_LEN 128
+#include <io.h>
 
 #if !defined(__linux__) || !defined(__x86_64__)
 #    error "Only supported in linux(x86_64)!"
 #endif
 
-int dynld_printf(const char* fmt, ...) {
-    va_list ap;
-    va_start(ap, fmt);
-    char buf[64];
-    int ret = dynld_vsnprintf(buf, sizeof(buf), fmt, ap);
-    va_end(ap);
-    syscall3(__NR_write, 1 /* stdout */, buf, ret);
-    return ret;
-}
-
-void entry(long* prctx) {
+void entry(const long* prctx) {
     // Interpret data on the stack passed by the OS kernel as specified in the
     // x86_64 SysV ABI.
 
diff --git a/include/io.h b/include/io.h
new file mode 100644
index 0000000..4b3ea4b
--- /dev/null
+++ b/include/io.h
@@ -0,0 +1,43 @@
+// Copyright (c) 2020 Johannes Stoelp
+
+#pragma once
+
+#include "fmt.h"
+#include "syscall.h"
+
+#include <asm/unistd.h>
+
+// `dynld_printf` uses fixed-size buffer on the stack for formating the message
+// (since we don't impl buffered I/O).
+//
+// Size can be re-configured by defining `MAX_PRINTF_LEN` before including
+// `io.h`.
+//
+// NOTE: This allows to specify an arbitrarily large buffer on the stack, but
+// for the purpose of this study that's fine, we are cautious.
+#if !defined(MAX_PRINTF_LEN)
+#    define MAX_PRINTF_LEN 64
+#endif
+
+#define FD_STDOUT 1
+#define FD_STDERR 2
+
+int dynld_printf(const char* fmt, ...) {
+    char buf[MAX_PRINTF_LEN];
+
+    va_list ap;
+    va_start(ap, fmt);
+    int ret = dynld_vsnprintf(buf, sizeof(buf), fmt, ap);
+    va_end(ap);
+
+    if (ret > MAX_PRINTF_LEN - 1) {
+        syscall3(__NR_write, FD_STDERR, buf, MAX_PRINTF_LEN - 1);
+
+        static const char warn[] = "\ndynld_printf: Message truncated, max length can be configured by defining MAX_PRINTF_LEN\n";
+        syscall3(__NR_write, FD_STDOUT, warn, sizeof(warn));
+        return MAX_PRINTF_LEN - 1;
+    }
+
+    syscall3(__NR_write, FD_STDOUT, buf, ret);
+    return ret;
+}