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Re: [Qemu-block] [PATCH] block/vvfat: Fix compiler warning with gcc 7

From: Hervé Poussineau
Subject: Re: [Qemu-block] [PATCH] block/vvfat: Fix compiler warning with gcc 7
Date: Mon, 17 Jul 2017 20:40:15 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

Le 17/07/2017 à 17:12, Max Reitz a écrit :
gcc 7 complains that the sprintf() might write a null byte beyond the
end of the tail buffer.  That is wrong, but we can silence it by making
i unsigned (it can never be negative anyway, see the if condition right
before).  For some reason, this allows gcc to suddenly accurately
calculate the range of i so we can give the tail[] array the exact size
it needs to have (which is 8 bytes) without gcc complaining.

In addition, let us convert the sprintf() to snprintf(), because that is
always nicer, and add an assertion about the range of the return value
afterwards so we can see that "8 - len" will never be negative and thus
"entry->name + MIN(j, 8 - len)" will never be out of bounds.

Signed-off-by: Max Reitz <address@hidden>
 block/vvfat.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/block/vvfat.c b/block/vvfat.c
index 6b11596..a9e207f 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -549,7 +549,7 @@ static direntry_t *create_short_filename(BDRVVVFATState *s,
     const gchar *p, *last_dot = NULL;
     gunichar c;
     bool lossy_conversion = false;
-    char tail[11];
+    char tail[8];

     if (!entry) {
         return NULL;
@@ -614,7 +614,8 @@ static direntry_t *create_short_filename(BDRVVVFATState *s,
     for (i = lossy_conversion ? 1 : 0; i < 999999; i++) {
         direntry_t *entry1;
         if (i > 0) {
-            int len = sprintf(tail, "~%d", i);
+            int len = snprintf(tail, sizeof(tail), "~%u", (unsigned)i);
+            assert(len <= 7);

As i is on minimum between 0 or 1 and on maximum equal at 999999, does it work 
if you change the type of i from int to unsigned int?
That way, you probably won't need the cast to unsigned in the s(n)printf.

             memcpy(entry->name + MIN(j, 8 - len), tail, len);
         for (entry1 = array_get(&(s->directory), directory_start);

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