By making sorting function for our ELF function list match on
both name and function, we ensure that the set of ELF functions
includes multiple copies for functions which have multiple instances
across CUs. For example, cpumask_weight has 22 instances in
System.map/kallsyms:
ffffffff8103b530 t cpumask_weight
ffffffff8103e300 t cpumask_weight
ffffffff81040d30 t cpumask_weight
ffffffff8104fa00 t cpumask_weight
ffffffff81064300 t cpumask_weight
ffffffff81082ba0 t cpumask_weight
ffffffff81084f50 t cpumask_weight
ffffffff810a4ad0 t cpumask_weight
ffffffff810bb740 t cpumask_weight
ffffffff8110a6c0 t cpumask_weight
ffffffff81118ab0 t cpumask_weight
ffffffff81129b50 t cpumask_weight
ffffffff81137dc0 t cpumask_weight
ffffffff811aead0 t cpumask_weight
ffffffff811d6800 t cpumask_weight
ffffffff811e1370 t cpumask_weight
ffffffff812fae80 t cpumask_weight
ffffffff81375c50 t cpumask_weight
ffffffff81634b60 t cpumask_weight
ffffffff817ba540 t cpumask_weight
ffffffff819abf30 t cpumask_weight
ffffffff81a7cb60 t cpumask_weight
With ELF representations for each address, and DWARF info about
addresses (low_pc) we can match DWARF with ELF accurately.
The result for the BTF representation is that we end up with
a single de-duped function:
[9287] FUNC 'cpumask_weight' type_id=9286 linkage=static
...and 22 DECL_TAGs for each address that point at it:
9288] DECL_TAG 'address=0xffffffff8103b530' type_id=9287 component_idx=-1
[9623] DECL_TAG 'address=0xffffffff8103e300' type_id=9287 component_idx=-1
[9829] DECL_TAG 'address=0xffffffff81040d30' type_id=9287 component_idx=-1
[11609] DECL_TAG 'address=0xffffffff8104fa00' type_id=9287 component_idx=-1
[13299] DECL_TAG 'address=0xffffffff81064300' type_id=9287 component_idx=-1
[15704] DECL_TAG 'address=0xffffffff81082ba0' type_id=9287 component_idx=-1
[15731] DECL_TAG 'address=0xffffffff81084f50' type_id=9287 component_idx=-1
[18582] DECL_TAG 'address=0xffffffff810a4ad0' type_id=9287 component_idx=-1
[20234] DECL_TAG 'address=0xffffffff810bb740' type_id=9287 component_idx=-1
[25384] DECL_TAG 'address=0xffffffff8110a6c0' type_id=9287 component_idx=-1
[25798] DECL_TAG 'address=0xffffffff81118ab0' type_id=9287 component_idx=-1
[26285] DECL_TAG 'address=0xffffffff81129b50' type_id=9287 component_idx=-1
[27040] DECL_TAG 'address=0xffffffff81137dc0' type_id=9287 component_idx=-1
[32900] DECL_TAG 'address=0xffffffff811aead0' type_id=9287 component_idx=-1
[35059] DECL_TAG 'address=0xffffffff811d6800' type_id=9287 component_idx=-1
[35353] DECL_TAG 'address=0xffffffff811e1370' type_id=9287 component_idx=-1
[48934] DECL_TAG 'address=0xffffffff812fae80' type_id=9287 component_idx=-1
[54476] DECL_TAG 'address=0xffffffff81375c50' type_id=9287 component_idx=-1
[87772] DECL_TAG 'address=0xffffffff81634b60' type_id=9287 component_idx=-1
[108841] DECL_TAG 'address=0xffffffff817ba540' type_id=9287 component_idx=-1
[132557] DECL_TAG 'address=0xffffffff819abf30' type_id=9287 component_idx=-1
[143689] DECL_TAG 'address=0xffffffff81a7cb60' type_id=9287 component_idx=-1
Consider another case where the same name - wakeup_show() - is
used for two different function signatures:
>From kernel/irq/irqdesc.c
static ssize_t wakeup_show(struct kobject *kobj,
struct kobj_attribute *attr, char *buf)
...and from drivers/base/power/sysfs.c
static ssize_t wakeup_show(struct device *dev, struct device_attribute *attr,
char *buf);
We see both defined in BTF, along with the addresses that
tell us which is which:
[28472] FUNC 'wakeup_show' type_id=11214 linkage=static
specifies
[11214] FUNC_PROTO '(anon)' ret_type_id=76 vlen=3
'kobj' type_id=877
'attr' type_id=11200
'buf' type_id=56
...and has declaration tag
[28473] DECL_TAG 'address=0xffffffff8115eab0' type_id=28472 component_idx=-1
which identifies
ffffffff8115eab0 t wakeup_show
...as the function with the first signature.
Similarly,
[114375] FUNC 'wakeup_show' type_id=4750 linkage=static
[4750] FUNC_PROTO '(anon)' ret_type_id=76 vlen=3
'dev' type_id=1488
'attr' type_id=3909
'buf' type_id=56
...and
[114376] DECL_TAG 'address=0xffffffff8181eac0' type_id=114375 component_idx=-1
...tell us that
ffffffff8181eac0 t wakeup_show
...has the second signature. So we can accommodate multiple
functions with conflicting signatures in BTF, since we have
added extra info to map from function description in BTF
to address.
In total for vmlinux 52006 DECL_TAGs are added, and these add
2MB to the BTF representation.
Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
---
btf_encoder.c | 26 ++++++++++++++++++++------
1 file changed, 20 insertions(+), 6 deletions(-)
diff --git a/btf_encoder.c b/btf_encoder.c
index 3bd0fe0..315053d 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -988,13 +988,25 @@ static int functions_cmp(const void *_a, const void *_b)
{
const struct elf_function *a = _a;
const struct elf_function *b = _b;
+ int ret;
/* if search key allows prefix match, verify target has matching
* prefix len and prefix matches.
*/
if (a->prefixlen && a->prefixlen == b->prefixlen)
- return strncmp(a->name, b->name, b->prefixlen);
- return strcmp(a->name, b->name);
+ ret = strncmp(a->name, b->name, b->prefixlen);
+ else
+ ret = strcmp(a->name, b->name);
+
+ if (ret || !b->addr)
+ return ret;
+
+ /* secondarily sort/search by address. */
+ if (a->addr < b->addr)
+ return -1;
+ if (a->addr > b->addr)
+ return 1;
+ return 0;
}
#ifndef max
@@ -1044,9 +1056,11 @@ static int btf_encoder__collect_function(struct btf_encoder *encoder, GElf_Sym *
}
static struct elf_function *btf_encoder__find_function(const struct btf_encoder *encoder,
- const char *name, size_t prefixlen)
+ struct function *fn, size_t prefixlen)
{
- struct elf_function key = { .name = name, .prefixlen = prefixlen };
+ struct elf_function key = { .name = function__name(fn),
+ .addr = fn->low_pc,
+ .prefixlen = prefixlen };
return bsearch(&key, encoder->functions.entries, encoder->functions.cnt, sizeof(key), functions_cmp);
}
@@ -1846,7 +1860,7 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu, struct co
continue;
/* prefer exact function name match... */
- func = btf_encoder__find_function(encoder, name, 0);
+ func = btf_encoder__find_function(encoder, fn, 0);
if (func) {
if (func->generated)
continue;
@@ -1863,7 +1877,7 @@ int btf_encoder__encode_cu(struct btf_encoder *encoder, struct cu *cu, struct co
* it does not have optimized-out parameters
* in any cu.
*/
- func = btf_encoder__find_function(encoder, name,
+ func = btf_encoder__find_function(encoder, fn,
strlen(name));
if (func) {
save = true;
--
2.31.1
