linux-kernel - Re: [PATCH v6 2/2] tools/nolibc: fix up size inflate regression

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Date:   Sun, 13 Aug 2023 11:00:37 +0200
From:   Willy Tarreau <w@....eu>
To:     Zhangjin Wu <falcon@...ylab.org>
Cc:     david.laight@...lab.com, arnd@...db.de,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        tanyuan@...ylab.org, thomas@...ch.de
Subject: Re: [PATCH v6 2/2] tools/nolibc: fix up size inflate regression

On Sat, Aug 12, 2023 at 05:51:53AM +0800, Zhangjin Wu wrote:
> As reported and suggested by Willy, the inline __sysret() helper
> introduces three types of conversions and increases the size:
> 
> (1) the "unsigned long" argument to __sysret() forces a sign extension
> from all sys_* functions that used to return 'int'
> 
> (2) the comparison with the error range now has to be performed on a
> 'unsigned long' instead of an 'int'
> 
> (3) the return value from __sysret() is a 'long' (note, a signed long)
> which then has to be turned back to an 'int' before being returned by the
> caller to satisfy the caller's prototype.
> 
> To fix up this, firstly, let's use macro instead of inline function to
> preserves the input type and avoids these useless conversions (1), (3).
> 
> Secondly, since all of the sys_* functions have been converted to return
> integer, now, it is able to remove comparison to a 'unsigned long'
> -MAX_ERRNO (2) and restore the simple sign comparison as before.
> 
(...)
> +/* Syscall return helper, set errno as -ret when ret < 0 */
> +#define __sysret(arg)                        \
> +({                                           \
> +	__typeof__(arg) __ret = (arg);       \
> +	if (__ret < 0) {                     \
> +		SET_ERRNO(-__ret);           \
> +		__ret = -1L;                 \
> +	}                                    \
> +	__ret;                               \
> +})

Except that this now breaks brk(), mmap() and sbrk() by taking any value
with MSB set as an error. Also you've re-introduced the problem you've
faced with const. See my simplification in the other thread by using "?:"
which does avoids any assignment.

Let's just roll brk(), mmap() and sbrk() to their original, working,
definition:

 static __attribute__((unused))
 void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset)
 {
        void *ret = sys_mmap(addr, length, prot, flags, fd, offset);
 
        if ((unsigned long)ret >= -MAX_ERRNO) {
                SET_ERRNO(-(long)ret);
                ret = MAP_FAILED;
        }
        return ret;
 }

And we're done, you can then keep the simplified __sysret() macro for all
other call places.

Willy