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SubjectANNOUNCE: pahole v1.13 (More BTF, __aligned__, __packed__)
Hi,

The v1.13 release of pahole and its friends is out, available at
the usual places:

Main git repo:

git://git.kernel.org/pub/scm/devel/pahole/pahole.git

Mirror git repo:

https://github.com/acmel/dwarves.git

tarball + gpg signature:

https://fedorapeople.org/~acme/dwarves/dwarves-1.13.tar.xz
https://fedorapeople.org/~acme/dwarves/dwarves-1.13.tar.sign

Here is a summary of changes:

- BTF

- Use of the recently introduced BTF deduplication algorithm present in the
Linux kernel's libbpf library, which allows for all the types in a multi
compile unit binary such as vmlinux to be compactly stored, without duplicates.

E.g.: from roughly:

$ readelf -SW ../build/v5.1-rc4+/vmlinux | grep .debug_info.*PROGBITS
[63] .debug_info PROGBITS 0000000000000000 1d80be0 c3c18b9 00 0 0 1
$
195 MiB

to:

$ time pahole --btf_encode ../build/v5.1-rc4+/vmlinux
real 0m19.168s
user 0m17.707s # On a Lenovo t480s (i7-8650U) SSD
sys 0m1.337s
$

$ readelf -SW ../build/v5.1-rc4+/vmlinux | grep .BTF.*PROGBITS
[78] .BTF PROGBITS 0000000000000000 27b49f61 1e23c3 00 0 0 1
$
~2 MiB

- Introduce a 'btfdiff' utility that prints the output from DWARF and from
BTF, comparing the pretty printed outputs, running it on various linux
kernel images, such as an allyesconfig for ppc64.

Running it on the above 5.1-rc4+ vmlinux:

$ btfdiff ../build/v5.1-rc4+/vmlinux
$

No differences from the types generated from the DWARF ELF sections to the
ones generated from the BTF ELF section.

- Add a BTF loader, i.e. 'pahole -F btf' allows pretty printing of structs
and unions in the same fashion as with DWARF info, and since BTF is way
more compact, using it is much faster than using DWARF.

$ cat ../build/v5.1-rc4+/vmlinux > /dev/null
$ perf stat -e cycles pahole -F btf ../build/v5.1-rc4+/vmlinux > /dev/null

Performance counter stats for 'pahole -F btf ../build/v5.1-rc4+/vmlinux':

229,712,692 cycles:u
0.063379597 seconds time elapsed
0.056265000 seconds user
0.006911000 seconds sys

$ perf stat -e cycles pahole -F dwarf ../build/v5.1-rc4+/vmlinux > /dev/null

Performance counter stats for 'pahole -F dwarf ../build/v5.1-rc4+/vmlinux':

49,579,679,466 cycles:u
13.063487352 seconds time elapsed
12.612512000 seconds user
0.426226000 seconds sys
$

- Better union support:

- Allow unions to be specified in pahole in the same fashion as structs

$ pahole -C thread_union ../build/v5.1-rc4+/net/ipv4/tcp.o
union thread_union {
struct task_struct task __attribute__((__aligned__(64))); /* 0 11008 */
long unsigned int stack[2048]; /* 0 16384 */
};
$

- Infer __attribute__((__packed__)) when structs have no alignment holes
and violate basic types (integer, longs, short integer) natural alignment
requirements. Several heuristics are used to infer the __packed__
attribute, see the changeset log for descriptions.

$ pahole -F btf -C boot_e820_entry ../build/v5.1-rc4+/vmlinux
struct boot_e820_entry {
__u64 addr; /* 0 8 */
__u64 size; /* 8 8 */
__u32 type; /* 16 4 */

/* size: 20, cachelines: 1, members: 3 */
/* last cacheline: 20 bytes */
} __attribute__((__packed__));
$

$ pahole -F btf -C lzma_header ../build/v5.1-rc4+/vmlinux
struct lzma_header {
uint8_t pos; /* 0 1 */
uint32_t dict_size; /* 1 4 */
uint64_t dst_size; /* 5 8 */

/* size: 13, cachelines: 1, members: 3 */
/* last cacheline: 13 bytes */
} __attribute__((__packed__));

- Support DWARF5's DW_AT_alignment, which, together with the __packed__
attribute inference algorithms produce output that, when compiled, should
produce structures with layouts that match the original source code.

See it in action with 'struct task_struct', which will also show some of the
new information at the struct summary, at the end of the struct:

$ pahole -C task_struct ../build/v5.1-rc4+/vmlinux | tail -19
/* --- cacheline 103 boundary (6592 bytes) --- */
struct vm_struct * stack_vm_area; /* 6592 8 */
refcount_t stack_refcount; /* 6600 4 */

/* XXX 4 bytes hole, try to pack */

void * security; /* 6608 8 */

/* XXX 40 bytes hole, try to pack */

/* --- cacheline 104 boundary (6656 bytes) --- */
struct thread_struct thread __attribute__((__aligned__(64))); /* 6656 4352 */

/* size: 11008, cachelines: 172, members: 207 */
/* sum members: 10902, holes: 16, sum holes: 98 */
/* sum bitfield members: 10 bits, bit holes: 2, sum bit holes: 54 bits */
/* paddings: 3, sum paddings: 14 */
/* forced alignments: 6, forced holes: 1, sum forced holes: 40 */
} __attribute__((__aligned__(64)));
$

- Add a '--compile' option to 'pfunct' that produces compileable output for the
function prototypes in an object file. There are still some bugs but the vast
majority of the kernel single compilation unit files the ones produced from a
single .c file are working, see the new 'fullcircle' utility that uses this
feature.

Example of it in action:

$ pfunct --compile=static_key_false ../build/v5.1-rc4+/net/ipv4/tcp.o
typedef _Bool bool;
typedef struct {
int counter; /* 0 4 */

/* size: 4, cachelines: 1, members: 1 */
/* last cacheline: 4 bytes */
} atomic_t;

struct jump_entry;

struct static_key_mod;


struct static_key {
atomic_t enabled; /* 0 4 */

/* XXX 4 bytes hole, try to pack */

union {
long unsigned int type; /* 8 8 */
struct jump_entry * entries; /* 8 8 */
struct static_key_mod * next; /* 8 8 */
}; /* 8 8 */

/* size: 16, cachelines: 1, members: 2 */
/* sum members: 12, holes: 1, sum holes: 4 */
/* last cacheline: 16 bytes */
};

bool static_key_false(struct static_key * key)
{
return *(bool *)1;
}

$

The generation of compilable code from the type information and its use in the
new tool 'fullcircle, helps validate all the parts of this codebase, finding
bugs that were lurking forever, go read the csets to find all sorts of curious
C language features that are rarely seen, like unnamed zero sized bitfields and
the way people have been using it over the years in a codebase like the linux
kernel.

Certainly there are several other features, changes and fixes that I forgot to
mention! Now lemme release this version so that we can use it more extensively
together with a recent patch merged for 5.2:

[PATCH bpf-next] kbuild: add ability to generate BTF type info for vmlinux

With it BTF will be always available for all the types of the kernel, which will
open a pandora box of cool new features that are in the works, and, for people
already using pahole, will greatly speed up its usage.

Please try to alias it to use btf, i.e.

alias pahole='pahole -F btf'

Please report any problems you may find with this new version or with the BTF
loader or any errors in the layout generated/pretty printed.

Thanks to the fine BTF guys at Facebook for the patches and help in testing,
fixing bugs and getting this out of the door, the stats for this release are:

Changesets: 157

113 Arnaldo Carvalho de Melo Red Hat
32 Andrii Nakryiko Facebook
10 Yonghong Song Facebook
1 Martin Lau Facebook
1 Domenico Andreoli

- Arnaldo

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