netdev - Re: [PATCH v2 1/2] bpf: extend stackmap to save binary_build

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Date:   Tue, 6 Mar 2018 18:38:36 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "hannes@...xchg.org" <hannes@...xchg.org>
Subject: Re: [PATCH v2 1/2] bpf: extend stackmap to save
 binary_build_id+offset instead of address



> On Mar 6, 2018, at 10:25 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Tue, Mar 06, 2018 at 10:09:13AM -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;
>> +	struct page *page;
>> +	int ret;
>> +
>> +	/*
>> +	 * vm_start is user memory, so we need to be careful with it.
>> +	 * We don't want too many copy_from_user to reduce overhead.
>> +	 * Most ELF file is at least one page long, and the build_id
>> +	 * is stored in the first page. Therefore, we limit the search of
>> +	 * build_id to the first page only. This can be made safe with
>> +	 * get_user_pages_fast(). If the file is smaller than PAGE_SIZE,
>> +	 * or the build_id is not in the first page, the look up fails.
>> +	 */
>> +	if (vma->vm_end - vma->vm_start < PAGE_SIZE ||
>> +	    vma->vm_start & (PAGE_SIZE - 1))	/* page aligned */
>> +		return -EINVAL;
>> +
>> +	if (get_user_pages_fast(vma->vm_start, 1, 0, &page) != 1)
>> +		return -EFAULT;
>> +
>> +	ret = -EINVAL;
>> +	ehdr = (Elf32_Ehdr *)vma->vm_start;
>> +
> 
> You're still directly accessing a user pointer afaict. This will
> insta-fault on SMAP enabled hardware due to the lack of STAC (or PAN on
> ARM).
> 
> You _really_ cannot just access user pointers without the proper APIs.
> 
> Did you maybe mean to use:
> 
> 	ehdr = (Elf32_Ehdr *)page_address(page);
> 
> ?

Yeah, I missed this part. Let me fix it. 

Thanks again!
Song


>> +	/* compare magic x7f "ELF" */
>> +	if (memcmp(ehdr->e_ident, ELFMAG, SELFMAG) != 0)
>> +		goto out;
>> +
>> +	/* only support executable file and shared object file */
>> +	if (ehdr->e_type != ET_EXEC && ehdr->e_type != ET_DYN)
>> +		goto out;
>> +
>> +	if (ehdr->e_ident[EI_CLASS] == 1)
>> +		ret = stack_map_get_build_id_32(vma, build_id);
>> +	else if (ehdr->e_ident[EI_CLASS] == 2)
>> +		ret = stack_map_get_build_id_64(vma, build_id);
>> +out:
>> +	put_page(page);
>> +	return ret;
>> +}