ANNOUNCE: pahole v1.26 (more holes, --bpf_features, --contains_enum)

Arnaldo Carvalho de Melo
2024-02-28T19:39:25Z

Hi,

The v1.26 release of pahole and its friends is out, showing more
holes (the ones in contained types) the ability to express the BTF
features to encode, to simplify the addition of new BTF features in the
Linux kernel build infrastructure, a way to find the enumeration with
some enumerator and various fixes.

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-1.26.tar.xz
https://fedorapeople.org/~acme/dwarves/dwarves-1.26.tar.bz2
https://fedorapeople.org/~acme/dwarves/dwarves-1.26.tar.sign

Thanks a lot to all the contributors and distro packagers, you're on the
CC list, I appreciate a lot the work you put into these tools,

Best Regards,

- Arnaldo

pahole:

- When expanding types using 'pahole -E' do it for union and struct typedefs and for enums too.

E.g: that 'state' field in 'struct module':

$ pahole module | head
struct module {
enum module_state state; /* 0 4 */

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

struct list_head list; /* 8 16 */
char name[56]; /* 24 56 */
/* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
struct module_kobject mkobj; /* 80 96 */
/* --- cacheline 2 boundary (128 bytes) was 48 bytes ago --- */
$

now gets expanded:

$ pahole -E module | head
struct module {
enum module_state {
MODULE_STATE_LIVE = 0,
MODULE_STATE_COMING = 1,
MODULE_STATE_GOING = 2,
MODULE_STATE_UNFORMED = 3,
} state; /* 0 4 */

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

$

- Print number of holes, bit holes and bit paddings in class member types.

Doing this recursively to show how much waste a complex data structure has
is something that still needs to be done, there were the low hanging fruits
on the path to having that feature.

For instance, for 'struct task_struct' in the Linux kernel we get this
extra info:

--- task_struct.before.c 2024-02-09 11:38:39.249638750 -0300
+++ task_struct.after.c 2024-02-09 16:19:34.221134835 -0300
@@ -29,6 +29,12 @@

/* --- cacheline 2 boundary (128 bytes) --- */
struct sched_entity se; /* 128 256 */
+
+ /* XXX last struct has 3 holes */
+
/* --- cacheline 6 boundary (384 bytes) --- */
struct sched_rt_entity rt; /* 384 48 */
struct sched_dl_entity dl; /* 432 224 */
+
+ /* XXX last struct has 1 bit hole */
+
/* --- cacheline 10 boundary (640 bytes) was 16 bytes ago --- */
const struct sched_class * sched_class; /* 656 8 */
struct rb_node core_node; /* 664 24 */
@@ -100,6 +103,9 @@
/* --- cacheline 35 boundary (2240 bytes) was 16 bytes ago --- */
struct list_head tasks; /* 2256 16 */
struct plist_node pushable_tasks; /* 2272 40 */
+
+ /* XXX last struct has 1 hole */
+
/* --- cacheline 36 boundary (2304 bytes) was 8 bytes ago --- */
struct rb_node pushable_dl_tasks; /* 2312 24 */
struct mm_struct * mm; /* 2336 8 */
@@ -172,6 +178,9 @@
/* XXX last struct has 4 bytes of padding */

struct vtime vtime; /* 2744 48 */
+
+ /* XXX last struct has 1 hole */
+
/* --- cacheline 43 boundary (2752 bytes) was 40 bytes ago --- */
atomic_t tick_dep_mask; /* 2792 4 */

@@ -396,9 +405,12 @@
/* --- cacheline 145 boundary (9280 bytes) --- */
struct thread_struct thread __attribute__((__aligned__(64))); /* 9280 4416 */

+ /* XXX last struct has 1 hole, 1 bit hole */
+
/* size: 13696, cachelines: 214, members: 262 */
/* sum members: 13518, holes: 21, sum holes: 162 */
/* sum bitfield members: 82 bits, bit holes: 2, sum bit holes: 46 bits */
/* member types with holes: 4, total: 6, bit holes: 2, total: 2 */
/* paddings: 6, sum paddings: 49 */
/* forced alignments: 2, forced holes: 2, sum forced holes: 88 */
};

- Introduce --contains_enumerator=ENUMERATOR_NAME:

E.g.:

$ pahole --contains_enumerator S_VERSION
enum file_time_flags {
S_ATIME = 1,
S_MTIME = 2,
S_CTIME = 4,
S_VERSION = 8,
}
$

The shorter form --contains_enum is also accepted.

- Fix pretty printing when using DWARF, where sometimes the class (-C) and a specified "type_enum",
may not be present on the same CU, so wait till both are found.

Now this example that reads the 'struct perf_event_header' and 'enum perf_event_type'
from the DWARF info in ~/bin/perf to pretty print records in the perf.data file works
just like when using type info from BTF in ~/bin/perf:

$ pahole -F dwarf -V ~/bin/perf \
--header=perf_file_header \
--seek_bytes '$header.data.offset' \
--size_bytes='$header.data.size' \
-C 'perf_event_header(sizeof,type,type_enum=perf_event_type,filter=type==PERF_RECORD_MMAP2)' \
--prettify perf.data --count 1
pahole: sizeof_operator for 'perf_event_header' is 'size'
pahole: type member for 'perf_event_header' is 'type'
pahole: type enum for 'perf_event_header' is 'perf_event_type'
pahole: filter for 'perf_event_header' is 'type==PERF_RECORD_MMAP2'
pahole: seek bytes evaluated from --seek_bytes=$header.data.offset is 0x3f0
pahole: size bytes evaluated from --size_bytes=$header.data.size is 0xd10
// type=perf_event_header, offset=0xc20, sizeof=8, real_sizeof=112
{
.header = {
.type = PERF_RECORD_MMAP2,
.misc = 2,
.size = 112,
},
.pid = 1533617,
.tid = 1533617,
.start = 94667542700032,
.len = 90112,
.pgoff = 16384,{
.maj = 0,
.min = 33,
.ino = 35914923,
.ino_generation = 26870,
},{
.build_id_size = 0,
.__reserved_1 = 0,
.__reserved_2 = 0,
.build_id = { 33, 0, 0, 0, -85, 4, 36, 2, 0, 0, 0, 0, -10, 104, 0, 0, 0, 0, 0, 0 },
},
.prot = 5,
.flags = 2,
.filename = "/usr/bin/ls",
},
$

DWARF loader:

- Add support for DW_TAG_constant, first seen in Go DWARF.

- Fix loading DW_TAG_subroutine_type generated by the Go compiler, where it may
have a DW_AT_byte_size. Go DWARF. And pretty print it as if
it was from C, this helped in writing BPF programs to attach to Go binaries, using
uprobes.

BTF loader:

- Fix loading of 32-bit signed enums.

BTF encoder:

- Add 'pahole --btf_features' to allow consumers to specify an opt-in set of
features they want to use in BTF encoding.

Supported features are a comma-separated combination of

encode_force Ignore invalid symbols when encoding BTF.
var Encode variables using BTF_KIND_VAR in BTF.
float Encode floating-point types in BTF.
decl_tag Encode declaration tags using BTF_KIND_DECL_TAG.
type_tag Encode type tags using BTF_KIND_TYPE_TAG.
enum64 Encode enum64 values with BTF_KIND_ENUM64.
optimized_func Encode representations of optimized functions
with suffixes like ".isra.0" etc
consistent_func Avoid encoding inconsistent static functions.
These occur when a parameter is optimized out
in some CUs and not others, or when the same
function name has inconsistent BTF descriptions
in different CUs.

Specifying "--btf_features=all" is the equivalent to setting all of the
above. If pahole does not know about a feature specified in
--btf_features it silently ignores it.

The --btf_features can either be specified via a single comma-separated
list
--btf_features=enum64,float

...or via multiple --btf_features values

--btf_features=enum64 --btf_features=float

These properties allow us to use the --btf_features option in the kernel
scripts/pahole_flags.sh script to specify the desired set of BTF
features.

If a feature named in --btf_features is not present in the version of
pahole used, BTF encoding will not complain. This is desired because it
means we no longer have to tie new features to a specific pahole
version.

Use --btf_features_strict to change that behaviour and bail out if one of
the requested features isn't present.

To see the supported features, use:

$ pahole --supported_btf_features
encode_force,var,float,decl_tag,type_tag,enum64,optimized_func,consistent_func
$

btfdiff:

- Parallelize loading BTF and DWARF, speeding up a bit.

- Do type expansion to cover "private" types and enumerations.