From f3475a4a22dd84be0d2d7daa11676ac861da64bc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureauls?= <marcandre.lureau@redhat.com>
Date: Tue, 14 Apr 2020 18:51:39 +0800
Subject: [PATCH] util: add slirp_fmt() helpers
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Various calls to snprintf() in libslirp assume that snprintf() returns
"only" the number of bytes written (excluding terminating NUL).

https://pubs.opengroup.org/onlinepubs/9699919799/functions/snprintf.html#tag_16_159_04

"Upon successful completion, the snprintf() function shall return the
number of bytes that would be written to s had n been sufficiently
large excluding the terminating null byte."

Introduce slirp_fmt() that handles several pathological cases the
way libslirp usually expect:

- treat error as fatal (instead of silently returning -1)

- fmt0() will always \0 end

- return the number of bytes actually written (instead of what would
have been written, which would usually result in OOB later), including
the ending \0 for fmt0()

- warn if truncation happened (instead of ignoring)

Other less common cases can still be handled with strcpy/snprintf() etc.
Signed-off-by: default avatarMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Samuel Thibault's avatarSamuel Thibault <samuel.thibault@ens-lyon.org>
Message-Id: <20200127092414.169796-2-marcandre.lureau@redhat.com>
---
 slirp/src/util.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++
 slirp/src/util.h |  3 +++
 2 files changed, 66 insertions(+)

diff --git a/slirp/src/util.c b/slirp/src/util.c
index e5960871..dcae899e 100644
--- a/slirp/src/util.c
+++ b/slirp/src/util.c
@@ -364,3 +364,66 @@ void slirp_pstrcpy(char *buf, int buf_size, const char *str)
     }
     *q = '\0';
 }
+
+static int slirp_vsnprintf(char *str, size_t size,
+                           const char *format, va_list args)
+{
+    int rv = vsnprintf(str, size, format, args);
+
+    if (rv < 0) {
+        g_error("vsnprintf() failed: %s", g_strerror(errno));
+    }
+
+    return rv;
+}
+
+/*
+ * A snprintf()-like function that:
+ * - returns the number of bytes written (excluding optional \0-ending)
+ * - dies on error
+ * - warn on truncation
+ */
+int slirp_fmt(char *str, size_t size, const char *format, ...)
+{
+    va_list args;
+    int rv;
+
+    va_start(args, format);
+    rv = slirp_vsnprintf(str, size, format, args);
+    va_end(args);
+
+    if (rv > size) {
+        g_critical("vsnprintf() truncation");
+    }
+
+    return MIN(rv, size);
+}
+
+/*
+ * A snprintf()-like function that:
+ * - always \0-end (unless size == 0)
+ * - returns the number of bytes actually written, including \0 ending
+ * - dies on error
+ * - warn on truncation
+ */
+int slirp_fmt0(char *str, size_t size, const char *format, ...)
+{
+    va_list args;
+    int rv;
+
+    va_start(args, format);
+    rv = slirp_vsnprintf(str, size, format, args);
+    va_end(args);
+
+    if (rv >= size) {
+        g_critical("vsnprintf() truncation");
+        if (size > 0)
+            str[size - 1] = '\0';
+        rv = size;
+    } else {
+        rv += 1; /* include \0 */
+    }
+
+    return rv;
+}
+
diff --git a/slirp/src/util.h b/slirp/src/util.h
index 3c6223ce..0558dfc2 100644
--- a/slirp/src/util.h
+++ b/slirp/src/util.h
@@ -177,4 +177,7 @@ static inline int slirp_socket_set_fast_reuse(int fd)
 
 void slirp_pstrcpy(char *buf, int buf_size, const char *str);
 
+int slirp_fmt(char *str, size_t size, const char *format, ...);
+int slirp_fmt0(char *str, size_t size, const char *format, ...);
+
 #endif
-- 
2.23.0