netdev - Re: [PATCH v3 1/2] bpf: extend stackmap to save binary_build

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Date:   Sat, 10 Mar 2018 00:25:11 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Song Liu <songliubraving@...com>, netdev@...r.kernel.org,
        ast@...nel.org, peterz@...radead.org
Cc:     kernel-team@...com, hannes@...xchg.org
Subject: Re: [PATCH v3 1/2] bpf: extend stackmap to save
 binary_build_id+offset instead of address

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