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From 9df2d784439720abbf67fa96c6515a5c4a9f230a Mon Sep 17 00:00:00 2001
From: Keith Packard <keithp@keithp.com>
Date: Thu, 14 Jan 2021 18:48:44 -0800
Subject: [PATCH 2/2] tinystdio: Fix snprintf(buf, 0, ...) to not smash buffer
snprintf(buf, 0) should not write anything to the destination, not
even a trailing '\0'. The tinystdio implementation had a signed
comparison bug where this case would cause a null to be placed in the
output buffer at the size of the data that would have been written.
Add a test to make sure snprintf respects the 'len' parameter
correctly.
Signed-off-by: Keith Packard <keithp@keithp.com>
---
newlib/libc/tinystdio/snprintf.c | 2 +-
test/printf_scanf.c | 31 +++++++++++++++++++++++++++++++
2 files changed, 32 insertions(+), 1 deletion(-)
--- a/newlib/libc/tinystdio/snprintf.c
+++ b/newlib/libc/tinystdio/snprintf.c
@@ -56,7 +56,7 @@
i = vfprintf(&f.file, fmt, ap);
va_end(ap);
- if (n >= 0 && i >= 0)
+ if ((int) n >= 0 && i >= 0)
s[i < n ? i : n] = 0;
return i;
--- a/test/printf_scanf.c
+++ b/test/printf_scanf.c
@@ -96,6 +96,37 @@
fflush(stdout);
}
#endif
+
+ /*
+ * test snprintf to make sure it doesn't overwrite the specified buffer
+ * length (even if that is zero)
+ */
+ for (x = 0; x <= 6; x++) {
+ char tbuf[10] = "xxxxxxxxx";
+ const char ref[10] = "xxxxxxxxx";
+ int i = snprintf(tbuf, x, "%s", "123");
+ int y = x <= 4 ? x : 4;
+ if (i != 3) {
+ printf("snprintf(tbuf, %d, \"%%s\", \"123\") return %d instead of %d\n",
+ x, i, 3);
+ errors++;
+ }
+ int l = strlen(tbuf);
+ if (y > 0 && l != y - 1) {
+ printf("returned buffer len want %d got %d\n", y - 1, l);
+ errors++;
+ }
+ if (y > 0 && strncmp(tbuf, "123", y - 1) != 0) {
+ strncpy(buf, "123", y - 1);
+ buf[y-1] = '\0';
+ printf("returned buffer want %s got %s\n", buf, tbuf);
+ errors++;
+ }
+ if (memcmp(tbuf + y, ref + y, sizeof(tbuf) - y) != 0) {
+ printf("tail of buf mangled %s\n", tbuf + y);
+ errors++;
+ }
+ }
for (x = 0; x < 32; x++) {
unsigned int v = 0x12345678 >> x;
unsigned int r;
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