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x86_64 Auxillary vector reports platform as i686 which doesn't match the linux kernel
Description of problem:
Based on the kernel source in the auxiliary vector AT_PLATFORM should be `x86_64` (confirmed by running outside qemu). However qemu sets it to `i686`.
This was originally reported with docker-for-mac, but was reduced on `x86_64` which is why it is pointless
Steps to reproduce:
1. Compile the following for x86_64 (statically if you don't want have an x86_64 dynamic linker) (code originally from https://stackoverflow.com/questions/26520163/accessing-auxiliary-vectors-c)
```
#include <stdio.h>
#include <elf.h>
int main(int argc, char** argv, char* envp[]) {
Elf64_auxv_t *auxv;
while(*envp++ != NULL);
/*from stack diagram above: *envp = NULL marks end of envp*/
int i = 0 ;
for (auxv = (Elf64_auxv_t *)envp; auxv->a_type != AT_NULL; auxv++)
/* auxv->a_type = AT_NULL marks the end of auxv */
{
if( auxv->a_type == AT_PLATFORM)
printf("AT_PLATFORM is: %s\n", ((char*)auxv->a_un.a_val));
}
}
```
2. Run with `qemu-x86_64-static`
3. See `AT_PLATFORM is: i686`
4. Compare to "real" x86_64 bit system which gives `AT_PLATFORM is: x86_64`
Additional information:
I think that adding `#define ELF_PLATFORM "x86_64"` [here](https://gitlab.com/qemu-project/qemu/-/blob/master/linux-user/elfload.c#L134) should work (but I don't fully understand the code). Otherwise we just end up getting the 32-bit case.
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