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Message-ID: <b97a8b35-4a67-4b75-8e18-45211e7afda7@gmail.com>
Date: Sat, 11 May 2024 17:11:05 +0200
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Subject: Re: Fix passing fcntl and ioctl variadic arguments to file system
 extension API
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To: djgpp AT delorie DOT com, Pali <pali AT pali DOT im>
References: <20240509182019 DOT mrbfcyv4jhl3w34x AT pali> <867cg03azy DOT fsf AT gnu DOT org>
From: "J.W. Jagersma (jwjagersma AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
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On 2024-05-11 12:21, Eli Zaretskii (eliz AT gnu DOT org) [via djgpp AT delorie DOT com] wrote:
>> Date: Thu, 9 May 2024 20:20:19 +0200
>> From: Pali <pali AT pali DOT im>
>> Cc: djgpp AT delorie DOT com
>>
>> Hello. I have there a change which fixes ioctl and fcntl functions when
>> file descriptor uses file system extension API. These two functions are
>> takes variadic arguments and file system extension API takes va_list
>> structure with all corresponding arguments. djgpp code currently do not
>> pass all arguments to API functions __FSEXT_ioctl and __FSEXT_fcntl.
>>
>> My fix, which is below, uses __builtin_apply_args() which returns
>> pointer to va_list-compatible structure of all arguments (including
>> non-variadic). It works for djgpp i386 calling convention. It does not
>> work on e.g. AMD64, but this is not a djgpp case which is 32-bit x86.
>> For using __builtin_apply_args, it was required to manually inline
>> __FSEXT_func_wrapper function as it takes variadic arguments and not
>> va_list structure as the "func" itself.

I (still) can't see Pali's message but this sounds similar to a patch I
sent in last year:

https://www.delorie.com/djgpp/mail-archives/browse.cgi?p=djgpp/2023/06/28/12:14:52

At the moment, fsext-hooked ioctl() and fnctl() are useless and always
invoke UB (entirely random effects), so I do think this is a
high-priority bug.

Linked patch is technically UB too (sometimes reading past the end of va_args),
but in that case, there is (should be) no code path where the va_args are used.

However I wasn't aware of __builtin_apply_args().  Whatever Pali's
solution looks like, it is probably nicer than mine :)