Fwd: [PATCH] coreutils/printf: fix errno clobbering with glibc 2.43

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Because moderated by too many recipients


On Mon, 30 Mar 2026 at 00:34, Roberto A. Foglietta
<[email protected]> wrote:
>
> On Sun, 29 Mar 2026 at 23:38, David Laight <[email protected]> wrote:
> >
> > On Sun, 29 Mar 2026 23:10:37 +0200
> > "Roberto A. Foglietta" <[email protected]> wrote:
> >
>
> [...]
>
> > > Relying only on a check of negative return value from printf() is a
> > > too simple solution? Just asking, not humor.
> >
> > It doesn't work...
> > Try:
> >         printf "fubar" >/dev/full
> >
> > Additionally even if printf() does return -1, errno is undefined.
> >
> > You need to call fflush() to get the buffered data written in order
> > to get an error reported if the underlying write() system call fails.
> >
> > Which is why the correct solution is to ignore the return value from
> > printf(). Instead call fflush(stdoout) and ferrror(stdout) before
> > exiting.
>
> ...but flushing changes the behaviour of printf and impact on stderr
> as well even when it is not involved. Unless the applet flush when
> exit()  because exit() would do anyway but doing first allows it to
> catch the errors. ... but also exit() is not granted in an embedded
> system that theoretically could run always as pid=1 and using alway
> built-in applications without never exiting. (self-contained case).
> Ignoring ENOENT everytime printf() size is not zero-length and avoid
> it when it is zero length? The latter is a change in behavior (also)
> but it can be seen as an optimization (step value to zero, not linear
> optimization).
>

This should filter out the glibc dirty errno false positive (it is
interesting that glibc asks busybox to fix a regression they create...
lol). In fact, after having understood the issue, the best thing is that
glibc clear errno when they know it is just a dirty value.

https://github.com/robang74/busybox/tree/printf

diff --git a/coreutils/printf.c b/coreutils/printf.c
index f7d98debc..5ddcee167 100644
--- a/coreutils/printf.c
+++ b/coreutils/printf.c
@@ -240,9 +240,11 @@ static int print_direc(char *format, unsigned fmt_length,
                        /* Hope compiler will optimize it out by moving call
                         * instruction after the ifs... */
                        if (!have_width) {
-                               if (!have_prec)
+                               if (!have_prec) {
+                                   ret = 1;
+                                   if(argument[0])
                                        ret = printf(format, argument,
/*unused:*/ argument, argument);
-                               else
+                               } else
                                        ret = printf(format,
precision, argument, /*unused:*/ argument);
                        } else {
                                if (!have_prec)
@@ -377,15 +379,20 @@ static char **print_formatted(char *f, char
**argv, int *conv_err)
                                } else {
                                        p = NULL;
                                }
+                               #define ret precision
                                if (*argv) {
-                                       *conv_err =
+                                       ret =
                                        print_direc(direc_start,
direc_length, field_width,
-
precision, *argv++) < 0 ? 1 : 0;
+
precision, *argv++);
                                } else {
-                                       *conv_err =
+                                       ret =
                                        print_direc(direc_start,
direc_length, field_width,
-
precision, "") < 0 ? 1 : 0;
+                                                               precision, "");
                                }
+                               /* here ret cannot be zero unless a
mistake happens
+                                * because print_direc() is changed to
skip "%s",""
+                                */
+                               *conv_err |= (ret > 0) ? (errno &&
errno != ENOENT) : errno;
                                free(p);
                        }
                        break;



Best regards, R-