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Turns out that btf_dump API doesn't handle a bunch of tricky corner cases, as reported by Per, and further discovered using his testing Python script ([0]). This patch revamps btf_dump's padding logic significantly, making it more correct and also avoiding unnecessary explicit padding, where compiler would pad naturally. This overall topic turned out to be very tricky and subtle, there are lots of subtle corner cases. The comments in the code tries to give some clues, but comments themselves are supposed to be paired with good understanding of C alignment and padding rules. Plus some experimentation to figure out subtle things like whether `long :0;` means that struct is now forced to be long-aligned (no, it's not, turns out). Anyways, Per's script, while not completely correct in some known situations, doesn't show any obvious cases where this logic breaks, so this is a nice improvement over the previous state of this logic. Some selftests had to be adjusted to accommodate better use of natural alignment rules, eliminating some unnecessary padding, or changing it to `type: 0;` alignment markers. Note also that for when we are in between bitfields, we emit explicit bit size, while otherwise we use `: 0`, this feels much more natural in practice. Next patch will add few more test cases, found through randomized Per's script. [0] https://lore.kernel.org/bpf/85f83c333f5355c8ac026f835b18d15060725fcb.camel@ericsson.com/ Reported-by: Per Sundström XP <per.xp.sundstrom@ericsson.com> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20221212211505.558851-6-andrii@kernel.org |
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