Re: bug in __gmp_replacement_vsnprintf

[Vincent Lefevre]

On 2023-08-21 10:59:13 +0200, Paul Zimmermann wrote:
> here is a small program that exhibits the bug (for example on gcc231):
[...]

An OpenBSD 7 machine, and this is due to the fact that %n is
no longer supported by the system *printf functions, so that
GMP uses __gmp_replacement_vsnprintf.

See also in the archives:

https://gmplib.org/list-archives/gmp-bugs/2022-October/005200.html
https://gmplib.org/list-archives/gmp-bugs/2023-January/005230.html

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Re: bug in __gmp_replacement_vsnprintf

[Niels Möller]

Paul Zimmermann  writes:

> this bug report got no feedback so far:
>
> https://gmplib.org/list-archives/gmp-bugs/2022-October/005200.html
>
> Do the GMP developers acknowledge it?

I'm not so familiar with this part of GMP, but it looks like a bug to
me.

I would suggest first changing the ASSERT at
https://gmplib.org/repo/gmp/file/tip/printf/repl-vsnprintf.c#l355 to
ASSERT_ALWAYS; it seems rather dangerous to pass format specifiers we're
not understanding to the system's vsprintf. And to really fix this
issue, we'd also have to actually support hex floats with %a and %A.

Looks like the file was written by Kevin Ryde two decades ago. I wonder
if it's possible/reasonable to replace with gnulib's version? That's a
few thousand lines of rather non-trivial code, though:
https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=lib/vasnprintf.c;h=ab11ad026ed1b5d224dd71a8c880a30d859339cb;hb=HEAD

Regards,
/Niels

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