[git commit] echo: do not assume that free() leaves errno unmodified

[Denys Vlasenko]

commit: https://git.busybox.net/busybox/commit/?id=e880c9c100028e6b0e805d4637139c67aea83748
branch: https://git.busybox.net/busybox/commit/?id=refs/heads/master

musl libc's mallocng free() may modify errno if kernel does not support
MADV_FREE which causes echo to echo with error when it shouldn't.

Future versions of POSIX[1] will require that free() leaves errno
unmodified but til then, do not rely free() implementation.

Should fix downstream issues:
https://github.com/alpinelinux/docker-alpine/issues/134
https://gitlab.alpinelinux.org/alpine/aports/-/issues/12311

Signed-off-by: Natanael Copa <ncopa at alpinelinux.org>
Signed-off-by: Denys Vlasenko <vda.linux at googlemail.com>
---
 coreutils/echo.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/coreutils/echo.c b/coreutils/echo.c
index b3828894c..61ba060ec 100644
--- a/coreutils/echo.c
+++ b/coreutils/echo.c
@@ -97,6 +97,7 @@ int echo_main(int argc UNUSED_PARAM, char **argv)
 #else
 	char nflag = 1;
 	char eflag = 0;
+	int err;
 
 	while ((arg = *++argv) != NULL) {
 		char n, e;
@@ -185,13 +186,12 @@ int echo_main(int argc UNUSED_PARAM, char **argv)
  do_write:
 	/* Careful to error out on partial writes too (think ENOSPC!) */
 	errno = 0;
-	/*r =*/ full_write(STDOUT_FILENO, buffer, out - buffer);
-	free(buffer);
-	if (/*WRONG:r < 0*/ errno) {
+	err = full_write(STDOUT_FILENO, buffer, out - buffer) != out - buffer;
+	if (err) {
 		bb_simple_perror_msg(bb_msg_write_error);
-		return 1;
 	}
-	return 0;
+	free(buffer);
+	return err;
 }
 
 /*