Hi Geert, On Fri, May 03, 2019 at 09:04:17AM +0200, Geert Uytterhoeven wrote:
Hi Baruch, On Fri, May 3, 2019 at 6:06 AM Baruch Siach <baruch@xxxxxxxxxx> wrote:strace 5.0 fails to build for m86k/5208 with the Buildroot generated toolchain: In file included from bpf_attr_check.c:6:0: static_assert.h:20:25: error: static assertion failed: "bpf_prog_info_struct.nr_jited_ksyms offset mismatch" # define static_assert _Static_assert ^ bpf_attr_check.c:913:2: note: in expansion of macro ‘static_assert’ static_assert(offsetof(struct bpf_prog_info_struct, nr_jited_ksyms) == offsetof(struct bpf_prog_info, nr_jited_ksyms), ^~~~~~~~~~~~~ The direct cause is a difference in the hole after the gpl_compatible field. Here is pahole output for the kernel struct (from v4.19): struct bpf_prog_info { ... __u32 ifindex; /* 80 4 */ __u32 gpl_compatible:1; /* 84: 0 4 */ /* XXX 15 bits hole, try to pack */ /* Bitfield combined with next fields */ __u64 netns_dev; /* 86 8 */I guess that should be "__aligned_u64 netns_dev;", to not rely on implicit alignment.
This is exactly the fix we suggested a year ago in https://lore.kernel.org/lkml/20180527112842.GA18204@xxxxxxxxxxxxxxxxx/T/#u when the issue arose for the first time, but, unfortunately, bpf maintainers took a more error-prone approach. I suppose our initial fix is still applicable.
And this is for the strace struct: struct bpf_prog_info_struct { ... uint32_t ifindex; /* 80 4 */ uint32_t gpl_compatible:1; /* 84: 0 4 */ /* XXX 31 bits hole, try to pack */How come the uint64_t below is 8-byte aligned, not 2-byte aligned? Does strace use a special definition of uint64_t?uint64_t netns_dev; /* 88 8 */ How should this be fixed?IMHO all "__u64" in structs tagged "__attribute__((aligned(8)))" should be replaced by "__aligned_u64", which is what the (whitespace-damaged) diff below does.
This is the approach taken in strace, see e.g. https://gitlab.com/strace/strace/blob/master/bpf_attr.h#L13 When <linux/bpf.h> in the kernel fails to set proper offsets, a static assertion in strace is triggered. -- ldv
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