Hi Geert,
On Fri, May 03 2019, Geert Uytterhoeven wrote:
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.
Thanks. I can confirm that this minimal change fixes strace build:
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 929c8e537a14..709d4dddc229 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2869,7 +2869,7 @@ struct bpf_prog_info {
char name[BPF_OBJ_NAME_LEN];
__u32 ifindex;
__u32 gpl_compatible:1;
- __u64 netns_dev;
+ __aligned_u64 netns_dev;
__u64 netns_ino;
__u32 nr_jited_ksyms;
__u32 nr_jited_func_lens;
Won't that break ABI compatibility for affected architectures?
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?
I guess this is because of the netns_dev field definition in struct
bpf_prog_info_struct at bpf_attr.h:
struct bpf_prog_info_struct {
...
uint32_t gpl_compatible:1;
/*
* The kernel UAPI is broken by Linux commit
* v4.16-rc1~123^2~227^2~5^2~2 .
*/
uint64_t ATTRIBUTE_ALIGNED(8) netns_dev; /* skip check */
Alignment is forced.
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.
Note that changing __bpf_md_ptr() may have a visible effect, as it is
used with struct bpf_sock, which has a size that is not a multiple of
8 bytes. So depending on architecture, a hole may appear.
baruch
--
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