[PATCH] tools/nolibc: make time_t robust if __kernel_old_time_t is missing in host headers

[PATCH] tools/nolibc: make time_t robust if __kernel_old_time_t is missing in host headers

[Zhouyi Zhou]

Commit 
d5094bcb5bfd ("tools/nolibc: define time_t in terms of __kernel_old_time_t")
made nolibc use the kernel's time type so that `time_t` matches
`timespec::tv_sec` on all ABIs (notably x32).    

Some distributions (e.g. Ubuntu 20.04 on ppc64le with older
exported headers under /usr/include) do not expose
`__kernel_old_time_t`, causing nolibc builds that rely on host
headers to fail (I discovered this when doing RCU torture test in
PPC VM of Open Source Lab of Oregon State University).
        
Keep the new behavior, but add a small compatibility shim:
if `__kernel_old_time_t` is not available, fall back to
 `signed long`

This preserves the intent of d5094bcb5bfd while letting nolibc
build on systems with older exported headers.

Fixes: d5094bcb5bfd ("tools/nolibc: define time_t in terms of __kernel_old_time_t")
Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>    
---
 tools/include/nolibc/std.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/include/nolibc/std.h b/tools/include/nolibc/std.h
index ba950f0e7338..ea225a0b16e9 100644
--- a/tools/include/nolibc/std.h
+++ b/tools/include/nolibc/std.h
@@ -29,6 +29,10 @@ typedef unsigned long       nlink_t;
 typedef   signed long         off_t;
 typedef   signed long     blksize_t;
 typedef   signed long      blkcnt_t;
+#ifndef __kernel_old_time_t
+typedef   signed long        time_t;
+#else
 typedef __kernel_old_time_t  time_t;
+#endif
 
 #endif /* _NOLIBC_STD_H */
-- 
2.25.1

Re: [PATCH] tools/nolibc: make time_t robust if __kernel_old_time_t is missing in host headers

[Thomas Weißschuh]

Hi Zhouyi,

thanks for your report and patch!

On 2025-09-18 01:08:33+0000, Zhouyi Zhou wrote:
> Commit 
> d5094bcb5bfd ("tools/nolibc: define time_t in terms of __kernel_old_time_t")
> made nolibc use the kernel's time type so that `time_t` matches
> `timespec::tv_sec` on all ABIs (notably x32).    

FYI you don't need to keep the referenced commit message on a single
line and should reflow it like regular text:

Commit d5094bcb5bfd ("tools/nolibc: define time_t in terms of
__kernel_old_time_t") made nolibc use the kernel's time type so that
`time_t` matches `timespec::tv_sec` on all ABIs (notably x32).    

(Source: I did the same thing as you until Linus told me to stop)

> Some distributions (e.g. Ubuntu 20.04 on ppc64le with older
> exported headers under /usr/include) do not expose
> `__kernel_old_time_t`, causing nolibc builds that rely on host
> headers to fail (I discovered this when doing RCU torture test in
> PPC VM of Open Source Lab of Oregon State University).

Instead of mentioning Ubuntu 20.04, just explain that __kernel_old_time_t
is fairly new, notably from 2020 in commit 94c467ddb273 ("y2038: add
__kernel_old_timespec and __kernel_old_time_t")

> Keep the new behavior, but add a small compatibility shim:
> if `__kernel_old_time_t` is not available, fall back to
>  `signed long`
> 
> This preserves the intent of d5094bcb5bfd while letting nolibc
> build on systems with older exported headers.
> 
> Fixes: d5094bcb5bfd ("tools/nolibc: define time_t in terms of __kernel_old_time_t")
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>    
> ---
>  tools/include/nolibc/std.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/tools/include/nolibc/std.h b/tools/include/nolibc/std.h
> index ba950f0e7338..ea225a0b16e9 100644
> --- a/tools/include/nolibc/std.h
> +++ b/tools/include/nolibc/std.h
> @@ -29,6 +29,10 @@ typedef unsigned long       nlink_t;
>  typedef   signed long         off_t;
>  typedef   signed long     blksize_t;
>  typedef   signed long      blkcnt_t;
> +#ifndef __kernel_old_time_t
> +typedef   signed long        time_t;
> +#else
>  typedef __kernel_old_time_t  time_t;
> +#endif

I don't think this works as __kernel_old_time_t is never defined.
Instead we could switch to __kernel_time_t, which is the same as
__kernel_old_time_t and has existed for longer.

>
>  #endif /* _NOLIBC_STD_H */


Thomas