Re: [PATCH bpf-next 1/2] bpf: extend stackmap to save binary_build_id+offset instead of address

[Peter Zijlstra]

On Mon, Feb 26, 2018 at 09:49:22AM -0800, Song Liu wrote:

> +/* Parse build ID of ELF file mapped to vma */
> +static int stack_map_get_build_id(struct vm_area_struct *vma,
> +                               unsigned char *build_id)
> +{
> +     Elf32_Ehdr *ehdr = (Elf32_Ehdr *)vma->vm_start;

How does this work? Isn't vma->vm_start a _user_ address?

> +
> +     /* we need at least 1 file header and 1 program header */
> +     if (vma->vm_end - vma->vm_start <
> +         sizeof(Elf32_Ehdr) + sizeof(Elf32_Phdr))
> +             return -EINVAL;
> +
> +     /* compare magic x7f "ELF" */
> +     if (memcmp(ehdr->e_ident, ELFMAG, SELFMAG) != 0)
> +             return -EINVAL;

So how come you can just dereference it here, without
get_user()/copy_from_user() etc.. ?

> +
> +     /* only support executable file and shared object file */
> +     if (ehdr->e_type != ET_EXEC && ehdr->e_type != ET_DYN)
> +             return -EINVAL;
> +
> +     if (ehdr->e_ident[EI_CLASS] == 1)
> +             return stack_map_get_build_id_32(vma, build_id);
> +     else if (ehdr->e_ident[EI_CLASS] == 2)
> +             return stack_map_get_build_id_64(vma, build_id);
> +     return -EINVAL;
> +}