Re: [musl] issue with exp10l

[Rich Felker <dalias@libc.org>]

On Thu, Jan 07, 2021 at 12:17:33PM +0100, Paul Zimmermann wrote:
>        Hi,
> 
> I am extending my comparison of the accuracy of several mathematical libraries
> to the "double extended precision" (long double on x86_64).
> 
> First I notice that Musl does not provide j0, j1, y0, and y1 for the long
> double format. Do you confirm?

I believe that's correct; they're not part of the standard and don't
seem to be an extension we implement at this time.

> Then I got a segmentation fault using exp10l with NaN input with a non-zero
> payload.
> 
> $ cat test_exp10.c
> #define _GNU_SOURCE
> 
> typedef union { __uint128_t n; long double x; } union_t;
> 
> #include <stdio.h>
> #include <math.h>
> int main()
> {
>   union_t u;
>   u.n = 16383UL;
>   u.n = u.n << 64;
>   u.n = u.n | 629329181547216221UL;
>   /* u.n = 302213637488765131341149 */
>   long double x = u.x;
>   printf ("x=%La\n", x);
>   fflush (stdout);
>   long double y;
>   y = exp10l (x);
>   printf ("y=%La\n", y);
>   fflush (stdout);
>   return 0;
> }
> 
> With glibc this works fine:
> 
> $ gcc -fno-builtin test_exp10.c -lm
> $ ./a.out
> x=nan
> y=-nan
> 
> With Musl 1.2.1 I get:
> 
> $ ./a.out
> x=nan
> Segmentation fault
> 
> According to gdb, the issue is in pow10l:
> 
> Program received signal SIGSEGV, Segmentation fault.
> 0x000055555555d10e in pow10l ()
> (gdb) where
> #0  0x000055555555d10e in pow10l ()
> #1  0x0000000080000000 in ?? ()
> #2  0x0000000000003fff in ?? ()
> #3  0x0000000000000000 in ?? ()

I can't reproduce this; I get x=nan y=nan. Can you provide a
disassembly and register dump of the point of crash? Did you do
anything weird building musl, or are you using a stock build from a
distro or musl-cross-make?

Rich