[git commit] printf: fix format string sanity check

Denys Vlasenko vda.linux at googlemail.com
Tue Jul 18 14:00:38 UTC 2017

commit: https://git.busybox.net/busybox/commit/?id=513a2457b65894b10b9fd6aa8753fca59eced08c
branch: https://git.busybox.net/busybox/commit/?id=refs/heads/master

One of the tests for printf checks for an invalid bare '%' in the
format string:

   $ busybox printf '%' a b c
   printf: %: invalid format

On x86_64 a slightly different test doesn't work correctly:

   $ busybox printf '%' d e f
   printf: invalid number 'd'
   printf: invalid number 'e'
   printf: invalid number 'f'

On other platforms the test fails randomly depending on how the
arguments are laid out in memory.

There are two places in the code where strchr is used to determine if
a character in the format string is valid.  However, strchr also returns
a valid pointer if the character being searched for is the null terminator
thus causing the code to incorrectly suppose that a valid character has
been found.

Add explicit checks for the null terminator.

Signed-off-by: Ron Yorston <rmy at pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux at googlemail.com>
 coreutils/printf.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/coreutils/printf.c b/coreutils/printf.c
index bc22e0e..65bb5a9 100644
--- a/coreutils/printf.c
+++ b/coreutils/printf.c
@@ -305,7 +305,7 @@ static char **print_formatted(char *f, char **argv, int *conv_err)
-			if (strchr("-+ #", *f)) {
+			if (*f && strchr("-+ #", *f)) {
@@ -348,7 +348,7 @@ static char **print_formatted(char *f, char **argv, int *conv_err)
 				static const char format_chars[] ALIGN1 = "diouxXfeEgGcs";
 				char *p = strchr(format_chars, *f);
 				/* needed - try "printf %" without it */
-				if (p == NULL) {
+				if (p == NULL || *f == '\0') {
 					bb_error_msg("%s: invalid format", direc_start);
 					/* causes main() to exit with error */
 					return saved_argv - 1;

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