Hi.

On Thu, 4 Feb 2021 at 04:13, Andrii Nakryiko [off-list ref] wrote:

On Mon, Feb 1, 2021 at 9:26 AM Giuliano Procida [off-list ref] wrote:

quoted

pahole -J needs to do the following to an ELF file:

* add or update the ".BTF" section
* maybe update the section name string table
* update the Section Header Table (SHT)

libelf either takes full control of layout or requires the user to
specify offset, size and alignment of all new and updated sections and
headers.

To avoid libelf moving program segments in particular, we position the

It's not clear to me what's wrong with libelf handling all the layout.
Even if libelf will move program segments around, what's the harm?
Does it break anything if we just let libelf do this?

It doesn't hurt the userspace case I care about. I've no idea what it
means in terms of vmlinux.

However, I wrote that text before I discovered that pahole -J isn't
actually used to modify kernel images.

One thing I haven't tried is to try to make .BTF loadable but leave
placement to libelf.

quoted

".BTF" and section name string table (typically named ".shstrtab")
sections after all others. The SHT always lives at the end of the file.

Note that the last section in an ELF file is normally the section name
string table and any ".BTF" section will normally be second last.
However, if these sections appear earlier, then we'll waste some space
in the ELF file when we rewrite them.

Signed-off-by: Giuliano Procida <redacted>
---
 libbtf.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 62 insertions(+), 2 deletions(-)

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