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

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Message-ID: <9CBA3A22-7EFD-42E7-93BE-ADD219929311@fb.com>
Date:   Tue, 6 Mar 2018 18:08:10 +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 Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "riel@...riel.com" <riel@...riel.com>
Subject: Re: [PATCH bpf-next 1/2] bpf: extend stackmap to save
 binary_build_id+offset instead of address



> On Mar 5, 2018, at 8:26 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> 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;
>> +}

I will fix this in v2. 

Thanks!
Song