linux-kernel - Re: [RFC PATCH v4 10/29] bpf tools: Collect map definitions from 'maps' section

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Date:	Wed, 27 May 2015 19:28:33 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	paulus@...ba.org, a.p.zijlstra@...llo.nl, mingo@...hat.com,
	acme@...nel.org, namhyung@...nel.org, jolsa@...nel.org,
	dsahern@...il.com, daniel@...earbox.net, brendan.d.gregg@...il.com,
	masami.hiramatsu.pt@...achi.com, lizefan@...wei.com,
	linux-kernel@...r.kernel.org, pi3orama@....com
Subject: Re: [RFC PATCH v4 10/29] bpf tools: Collect map definitions from
 'maps' section

On Thu, May 28, 2015 at 10:03:04AM +0800, Wangnan (F) wrote:
> 
> 
> On 2015/5/28 9:53, Alexei Starovoitov wrote:
> >On Wed, May 27, 2015 at 05:19:45AM +0000, Wang Nan wrote:
> >>If maps are used by eBPF programs, corresponding object file(s) should
> >>contain a section named 'map'. Which contains map definitions. This
> >>patch copies the data of the whole section. Map data parsing should be
> >>acted just before map loading.
> >>
> >>Signed-off-by: Wang Nan <wangnan0@...wei.com>
> >>---
> >...
> >>+static int
> >>+bpf_object__init_maps(struct bpf_object *obj, void *data,
> >>+		      size_t size)
> >>+{
> >>+	if (size == 0) {
> >>+		pr_debug("%s doesn't need map definition\n",
> >>+			 obj->path);
> >>+		return 0;
> >>+	}
> >>+
> >>+	obj->maps_buf = malloc(size);
> >>+	if (!obj->maps_buf) {
> >>+		pr_warning("malloc maps failed: %s\n", obj->path);
> >>+		return -ENOMEM;
> >>+	}
> >>+
> >>+	obj->maps_buf_sz = size;
> >>+	memcpy(obj->maps_buf, data, size);
> >why copy it? To create maps and apply fixups to instructions
> >relo sections are needed anyway, so elf has to be open while
> >this section is being processed. So why copy?
> >
> 
> When creating maps, ELF file has been closed.
> 
> I divide libelf info two phases: opening and loading. ELF file is closed
> at the end of opening phase. I think some caller need 'opening' phase only.
> For example, checking metadata in an eBPF object file. In this case, we
> don't
> need create map file descriptors.

loading elf into memory, parsing it, copying map, prog, relo sections
just to check metadata? That doesn't sound like real use case.
imo it's cleaner to remember where maps and relocations are in a loaded elf,
then create maps, patch copied progs and release all elf.
This elfs are all very small, so we're not talking about large memory savings,
but still.

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