Re: [PATCH v2 03/33] linux-user/x86_64: Allocate vsyscall page as a commpage

[Ilya Leoshkevich]

On Tue, 2022-08-16 at 15:33 -0500, Richard Henderson wrote:
> We're about to start validating PAGE_EXEC, which means that we've
> got to the vsyscall page executable.  We had been special casing
> this entirely within translate.
> 
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  linux-user/elfload.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index 29d910c4cc..d783240a36 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -195,6 +195,28 @@ static void
> elf_core_copy_regs(target_elf_gregset_t *regs, const CPUX86State *en
>      (*regs)[26] = tswapreg(env->segs[R_GS].selector & 0xffff);
>  }
>  
> +#if ULONG_MAX >= TARGET_VSYSCALL_PAGE
> +#define HI_COMMPAGE  TARGET_VSYSCALL_PAGE
> +
> +static bool init_guest_commpage(void)
> +{
> +    /*
> +     * The vsyscall page is at a high negative address aka kernel
> space,
> +     * which means that we cannot actually allocate it with
> target_mmap.
> +     * We still should be able to use page_set_flags, unless the
> user
> +     * has specified -R reserved_va, which would trigger an
> assert().
> +     */
> +    if (reserved_va != 0 &&
> +        TARGET_VSYSCALL_PAGE + TARGET_PAGE_SIZE >= reserved_va) {
> +        error_report("Cannot allocate vsyscall page");
> +        exit(EXIT_FAILURE);
> +    }
> +    page_set_flags(TARGET_VSYSCALL_PAGE,
> +                   TARGET_VSYSCALL_PAGE + TARGET_PAGE_SIZE,
> +                   PAGE_EXEC | PAGE_VALID);
> +    return true;
> +}
> +#endif
>  #else
>  
>  #define ELF_START_MMAP 0x80000000

I found this in context of wasmtime, but apparently the problem is more
broad: after this patch any x86_64 PIE binaries no longer run:

qemu-x86_64: ../linux-user/elfload.c:2657: pgb_dynamic: Assertion
`sizeof(uintptr_t) == 4' failed.
Aborted (core dumped)

(Maybe we need a test for this, PIE version of "hello world" will do.)

I wonder if we need this assert at all?
There is a comment that says that 64-bit hosts should have used
reserved_va, but what is the reasoning behind this restriction?
Without this assert, pgb_find_hole() finds a suitable hole just fine.