[Bug libc/31492] New: ARM ldp instruction trigger bus error when kernel open option CONFIG_IO_STRICT_DEVMEM

["luofengwc at qq dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31492

            Bug ID: 31492
           Summary: ARM ldp instruction trigger bus error when kernel open
                    option CONFIG_IO_STRICT_DEVMEM
           Product: glibc
           Version: 2.34
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: luofengwc at qq dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Hi, everyone, I encountered a runtime problem with bus errror caused by the ldp
instruction(ARM instruction).
On an aarch architecture Linux machine, when I turned on the option kernel
option CONFIG_IO_STRICT_DEVMEM, mapped the memory of /dev/mem using mmap, and
used the ldp instruction to read the data, a bus error occurred, but ldr and
ldrp ran normally.

Why do i think this problem is related to glibc because the ldp command is also
used in memcpy. when use memcpy to copy /dev/mem data, alos has bus error;

check kernel CONFIG_IO_STRICT_DEVMEM optoins
```
zcat /proc/config.gz | grep CONFIG_IO_STRICT_DEVMEM
CONFIG_IO_STRICT_DEVMEM=y
```
code example: demo.c

```
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>

typedef char u8;

int main() {
        off_t base = 9654697984;
        void *p;
        int fd;
        int len = 724;

        struct stat statbuf;
        off_t mmoffset;
        void *mmp;

        if ((fd = open("/dev/mem", O_RDONLY)) == -1)
        {
                printf("%d\n",__LINE__);
                return NULL;
        }

        if ((p = malloc(len)) == NULL)
        {
                printf("%d\n",__LINE__);
                return NULL;
        }

        if (fstat(fd, &statbuf) == -1)
        {
                printf("%d\n",__LINE__);
                return NULL;
        }


        mmoffset = base % sysconf(_SC_PAGESIZE);

        printf("0");
        mmp = mmap(NULL, mmoffset + len, PROT_READ, MAP_SHARED, fd, base -
mmoffset);
        if (mmp == MAP_FAILED)
                return 0;

        long output;

        __asm__(
                "ldrb w0, [x0]"
                : "=r" (output)
                : "r" (mmp+mmoffset)
        );

        __asm__(
                "ldr x0, [x0]"
                : "=r" (output)
                : "r" (mmp+mmoffset)
        );
        // ldp trigger bus error
        __asm__(
                "ldp x0, x1, [x0]"
                : "=r" (output)         
                : "r" (mmp+mmoffset)
        );


        if (munmap(mmp, mmoffset + len) == -1)
        {
                return NULL;
        }

        printf("\noutput = %ld\n",output);

  return 0;
}
```
compile command with none optimization

```
gcc -O0 demo.c  -g
```

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