echo: do not assume that free() leaves errno unmodified

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@alpinelinux.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files 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;
 }
 
 /*