How to print __u128_t in kernel?

How to print __u128_t in kernel?

[Dongliang Mu]

Hello everyone,

I encounter one object with __uint128_t type in the following code.
But I am not sure how to print the content of __uint128_t.

struct user_fpsimd_state {
__uint128_t vregs[32];
__u32 fpsr;
__u32 fpcr;
__u32 __reserved[2];
};

At first, I would like to use:
pr_alert("0x%lx %lx\n", i, (uint64_t)(fpreg.vregs[i] >> 64),
(uint64_t)(fpreg.vregs[i]));

However, gcc reports error with "-Werror" enabled:

In function ‘print_fpregs_in_coredump’:
error: iteration 32 invokes undefined behavior
[-Werror=aggressive-loop-optimizations]
   89 |   pr_alert("V%d 0x%lx %lx\n", i, (uint64_t)(fpreg.vregs[i] >> 64),

Any idea to eliminate this error?

--
My best regards to you.

     No System Is Safe!
     Dongliang Mu

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Re: How to print __u128_t in kernel?

[Philipp Hortmann]

On 9/18/21 3:35 PM, Dongliang Mu wrote:
> Hello everyone,
> 
> I encounter one object with __uint128_t type in the following code.
> But I am not sure how to print the content of __uint128_t.
> 
> struct user_fpsimd_state {
> __uint128_t vregs[32];
> __u32 fpsr;
> __u32 fpcr;
> __u32 __reserved[2];
> };
> 
> At first, I would like to use:
> pr_alert("0x%lx %lx\n", i, (uint64_t)(fpreg.vregs[i] >> 64),
> (uint64_t)(fpreg.vregs[i]));
> 
> However, gcc reports error with "-Werror" enabled:
> 
> In function ‘print_fpregs_in_coredump’:
> error: iteration 32 invokes undefined behavior
> [-Werror=aggressive-loop-optimizations]
>     89 |   pr_alert("V%d 0x%lx %lx\n", i, (uint64_t)(fpreg.vregs[i] >> 64),
> 
> Any idea to eliminate this error?
> 
> --
> My best regards to you.
> 
>       No System Is Safe!
>       Dongliang Mu
> 

Hi

I do like this example:
https://lwn.net/Articles/745261/

...
pr_err("%llx%llx\n", (u64) (val >> 64), (u64) val);
pr_err("%llx%llx\n", (u64) (v >> 64), (u64) v);
...

But I have not tried it.

Bye Philipp



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Re: How to print __u128_t in kernel?

[Bjørn Mork]

Philipp Hortmann <philipp.g.hortmann@gmail.com> writes:

> I do like this example:
> https://lwn.net/Articles/745261/
>
> ...
> pr_err("%llx%llx\n", (u64) (val >> 64), (u64) val);
> pr_err("%llx%llx\n", (u64) (v >> 64), (u64) v);
> ...
>

This will produce the wrong output unless at least one of bit 60 - 63 is
set and have a confusing leading zero unless one of the upper 64 bits is
set.

Running this test:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
        __uint128_t foo;

        if (argc < 3)
                return -1;
        foo = (__uint128_t)strtoull(argv[1], NULL, 0) << 64 | strtoull(argv[2], NULL, 0);
        fprintf(stderr, "foo=0x%llx%llx\n", (__uint64_t) (foo >> 64), (__uint64_t) foo);
        fprintf(stderr, "foo=0x%016llx%016llx\n", (__uint64_t) (foo >> 64), (__uint64_t) foo);
        return 0;
}


Produces:

bjorn@miraculix:~$ /tmp/x 1 0
foo=0x10
foo=0x00000000000000010000000000000000
bjorn@miraculix:~$ /tmp/x 1 2
foo=0x12
foo=0x00000000000000010000000000000002
bjorn@miraculix:~$ /tmp/x 0 2
foo=0x02
foo=0x00000000000000000000000000000002



Bjørn

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