Date:   Fri, 9 Mar 2018 23:34:50 +0000
From:   Song Liu <songliubraving@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kernel Team <Kernel-team@...com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        Teng Qin <qinteng@...com>
Subject: Re: [PATCH v3 1/2] bpf: extend stackmap to save
 binary_build_id+offset instead of address



> On Mar 9, 2018, at 3:25 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> 
> Hi Song,
> 
> On 03/06/2018 08:42 PM, Song Liu wrote:
> [...]> +/*
>> + * Parse build id from the note segment. This logic can be shared between
>> + * 32-bit and 64-bit system, because Elf32_Nhdr and Elf64_Nhdr are
>> + * identical.
>> + */
>> +static inline int stack_map_parse_build_id(void *vm_start,
>> +					   unsigned char *build_id,
>> +					   void *note_start,
>> +					   Elf32_Word note_size)
>> +{
>> +	Elf32_Word note_offs = 0, new_offs;
>> +
>> +	/* check for overflow */
>> +	if (note_start < vm_start || note_start + note_size < note_start)
>> +		return -EINVAL;
>> +
>> +	/* only supports note that fits in the first page */
>> +	if (note_start + note_size > vm_start + PAGE_SIZE)
>> +		return -EINVAL;
>> +
>> +	while (note_offs + sizeof(Elf32_Nhdr) < note_size) {
>> +		Elf32_Nhdr *nhdr = (Elf32_Nhdr *)(note_start + note_offs);
>> +
>> +		if (nhdr->n_type == BPF_BUILD_ID &&
>> +		    nhdr->n_namesz == sizeof("GNU") &&
>> +		    nhdr->n_descsz == BPF_BUILD_ID_SIZE) {
>> +			memcpy(build_id,
>> +			       note_start + note_offs +
>> +			       ALIGN(sizeof("GNU"), 4) + sizeof(Elf32_Nhdr),
>> +			       BPF_BUILD_ID_SIZE);
>> +			return 0;
>> +		}
>> +		new_offs = note_offs + sizeof(Elf32_Nhdr) +
>> +			ALIGN(nhdr->n_namesz, 4) + ALIGN(nhdr->n_descsz, 4);
>> +		if (new_offs <= note_offs)  /* overflow */
>> +			break;
>> +		note_offs = new_offs;
>> +	};
>> +	return -EINVAL;
>> +}
>> +
>> +/* Parse build ID from 32-bit ELF */
>> +static int stack_map_get_build_id_32(void *vm_start,
>> +				     unsigned char *build_id)
>> +{
>> +	Elf32_Ehdr *ehdr = (Elf32_Ehdr *)vm_start;
>> +	Elf32_Phdr *phdr;
>> +	int i;
>> +
>> +	/* only supports phdr that fits in one page */
>> +	if (ehdr->e_phnum >
>> +	    (PAGE_SIZE - sizeof(Elf32_Ehdr)) / sizeof(Elf32_Phdr))
>> +		return -EINVAL;
>> +
>> +	phdr = (Elf32_Phdr *)(vm_start + sizeof(Elf32_Ehdr));
>> +
>> +	for (i = 0; i < ehdr->e_phnum; ++i)
>> +		if (phdr[i].p_type == PT_NOTE)
>> +			return stack_map_parse_build_id(vm_start, build_id,
>> +					vm_start + phdr[i].p_offset,
>> +					phdr[i].p_filesz);
>> +	return -EINVAL;
>> +}
>> +
>> +/* Parse build ID from 64-bit ELF */
>> +static int stack_map_get_build_id_64(void *vm_start,
>> +				     unsigned char *build_id)
>> +{
>> +	Elf64_Ehdr *ehdr = (Elf64_Ehdr *)vm_start;
>> +	Elf64_Phdr *phdr;
>> +	int i;
>> +
>> +	/* only supports phdr that fits in one page */
>> +	if (ehdr->e_phnum >
>> +	    (PAGE_SIZE - sizeof(Elf64_Ehdr)) / sizeof(Elf64_Phdr))
>> +		return -EINVAL;
>> +
>> +	phdr = (Elf64_Phdr *)(vm_start + sizeof(Elf64_Ehdr));
>> +
>> +	for (i = 0; i < ehdr->e_phnum; ++i)
>> +		if (phdr[i].p_type == PT_NOTE)
>> +			return stack_map_parse_build_id(vm_start, build_id,
>> +					vm_start + phdr[i].p_offset,
>> +					phdr[i].p_filesz);
>> +	return -EINVAL;
>> +}
>> +
>> +/* 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)
> 
> Shouldn't this throw a splat? Implementations of get_user_pages_fast()
> call down_read() which has might_sleep() and we're under RCU read side
> here.
> 
>> +		return -EFAULT;
>> +
>> +	ret = -EINVAL;
>> +	ehdr = (Elf32_Ehdr *)page_address(page);
>> +
>> +	/* 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)
> 
> Minor nit: ehdr->e_ident[EI_CLASS] == ELFCLASS32
> 
>> +		ret = stack_map_get_build_id_32(page_address(page), build_id);
>> +	else if (ehdr->e_ident[EI_CLASS] == 2)
> 
> Minor nit: ehdr->e_ident[EI_CLASS] == ELFCLASS64
> 
>> +		ret = stack_map_get_build_id_64(page_address(page), build_id);
>> +out:
>> +	put_page(page);
>> +	return ret;
>> +}
>> +
>> +static int stack_map_get_build_id_offset(struct bpf_map *map,
>> +					 struct stack_map_bucket *bucket,
>> +					 u64 *ips, u32 trace_nr)
>> +{
>> +	int i;
>> +	struct vm_area_struct *vma;
>> +	struct bpf_stack_build_id *id_offs;
>> +	int err;
>> +	int successful_lookup = 0;
>> +
> [...]
> 
> Thanks,
> Daniel

Thanks Daniel! I will fix these. 

I also chatted with Teng about this, that we may miss some cases. I will
also include that fix in the next version. 

Song

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