On 11/15/24 3:36 AM, Alan Maguire wrote:
From: Eduard Zingerman <eddyz87@xxxxxxxxx>
Song Liu reported that a kernel func (perf_event_read()) cannot be traced
in certain situations since the func is not in vmlinux bTF. This happens
in kernels 6.4, 6.9 and 6.11 and the kernel is built with pahole 1.27.
The perf_event_read() signature in kernel (kernel/events/core.c):
static int perf_event_read(struct perf_event *event, bool group)
Adding '-V' to pahole command line, and the following error msg can be found:
skipping addition of 'perf_event_read'(perf_event_read) due to unexpected register used for parameter
Eventually the error message is attributed to the setting
(parm->unexpected_reg = 1) in parameter__new() function.
The following is the dwarf representation for perf_event_read():
0x0334c034: DW_TAG_subprogram
DW_AT_low_pc (0xffffffff812c6110)
DW_AT_high_pc (0xffffffff812c640a)
DW_AT_frame_base (DW_OP_reg7 RSP)
DW_AT_GNU_all_call_sites (true)
DW_AT_name ("perf_event_read")
DW_AT_decl_file ("/rw/compile/kernel/events/core.c")
DW_AT_decl_line (4641)
DW_AT_prototyped (true)
DW_AT_type (0x03324f6a "int")
0x0334c04e: DW_TAG_formal_parameter
DW_AT_location (0x007de9fd:
[0xffffffff812c6115, 0xffffffff812c6141): DW_OP_reg5 RDI
[0xffffffff812c6141, 0xffffffff812c6323): DW_OP_reg14 R14
[0xffffffff812c6323, 0xffffffff812c63fe): DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
[0xffffffff812c63fe, 0xffffffff812c6405): DW_OP_reg14 R14
[0xffffffff812c6405, 0xffffffff812c640a): DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value)
DW_AT_name ("event")
DW_AT_decl_file ("/rw/compile/kernel/events/core.c")
DW_AT_decl_line (4641)
DW_AT_type (0x0333aac2 "perf_event *")
0x0334c05e: DW_TAG_formal_parameter
DW_AT_location (0x007dea82:
[0xffffffff812c6137, 0xffffffff812c63f2): DW_OP_reg12 R12
[0xffffffff812c63f2, 0xffffffff812c63fe): DW_OP_GNU_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
[0xffffffff812c63fe, 0xffffffff812c640a): DW_OP_reg12 R12)
DW_AT_name ("group")
DW_AT_decl_file ("/rw/compile/kernel/events/core.c")
DW_AT_decl_line (4641)
DW_AT_type (0x03327059 "bool")
By inspecting the binary, the second argument ("bool group") is used
in the function. The following are the disasm code:
ffffffff812c6110 <perf_event_read>:
ffffffff812c6110: 0f 1f 44 00 00 nopl (%rax,%rax)
ffffffff812c6115: 55 pushq %rbp
ffffffff812c6116: 41 57 pushq %r15
ffffffff812c6118: 41 56 pushq %r14
ffffffff812c611a: 41 55 pushq %r13
ffffffff812c611c: 41 54 pushq %r12
ffffffff812c611e: 53 pushq %rbx
ffffffff812c611f: 48 83 ec 18 subq $24, %rsp
ffffffff812c6123: 41 89 f4 movl %esi, %r12d
<=========== NOTE that here '%esi' is used and moved to '%r12d'.
ffffffff812c6126: 49 89 fe movq %rdi, %r14
ffffffff812c6129: 65 48 8b 04 25 28 00 00 00 movq %gs:40, %rax
ffffffff812c6132: 48 89 44 24 10 movq %rax, 16(%rsp)
ffffffff812c6137: 8b af a8 00 00 00 movl 168(%rdi), %ebp
ffffffff812c613d: 85 ed testl %ebp, %ebp
ffffffff812c613f: 75 3f jne 0xffffffff812c6180 <perf_event_read+0x70>
ffffffff812c6141: 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:(%rax,%rax)
ffffffff812c614b: 0f 1f 44 00 00 nopl (%rax,%rax)
ffffffff812c6150: 49 8b 9e 28 02 00 00 movq 552(%r14), %rbx
ffffffff812c6157: 48 89 df movq %rbx, %rdi
ffffffff812c615a: e8 c1 a0 d7 00 callq 0xffffffff82040220 <_raw_spin_lock_irqsave>
ffffffff812c615f: 49 89 c7 movq %rax, %r15
ffffffff812c6162: 41 8b ae a8 00 00 00 movl 168(%r14), %ebp
ffffffff812c6169: 85 ed testl %ebp, %ebp
ffffffff812c616b: 0f 84 9a 00 00 00 je 0xffffffff812c620b <perf_event_read+0xfb>
ffffffff812c6171: 48 89 df movq %rbx, %rdi
ffffffff812c6174: 4c 89 fe movq %r15, %rsi
<=========== NOTE: %rsi is overwritten
......
ffffffff812c63f0: 41 5c popq %r12
<============ POP r12
ffffffff812c63f2: 41 5d popq %r13
ffffffff812c63f4: 41 5e popq %r14
ffffffff812c63f6: 41 5f popq %r15
ffffffff812c63f8: 5d popq %rbp
ffffffff812c63f9: e9 e2 a8 d7 00 jmp 0xffffffff82040ce0 <__x86_return_thunk>
ffffffff812c63fe: 31 c0 xorl %eax, %eax
ffffffff812c6400: e9 be fe ff ff jmp 0xffffffff812c62c3 <perf_event_read+0x1b3>
It is not clear why dwarf didn't encode %rsi in locations. But
DW_OP_GNU_entry_value(DW_OP_reg4 RSI) tells us that RSI is live at
the entry of perf_event_read(). So this patch tries to search
DW_OP_GNU_entry_value/DW_OP_entry_value location/expression so if
the expected parameter register matches the register in
DW_OP_GNU_entry_value/DW_OP_entry_value, then the original parameter
is not optimized.
For one of internal 6.11 kernel, there are 62498 functions in BTF and
perf_event_read() is not there. With this patch, there are 62552 functions
in BTF and perf_event_read() is included.
Reported-by: Song Liu <song@xxxxxxxxxx>
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
---
dwarf_loader.c | 106 ++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 83 insertions(+), 23 deletions(-)
diff --git a/dwarf_loader.c b/dwarf_loader.c
index ec8641b..4789967 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1157,16 +1157,90 @@ static struct template_parameter_pack *template_parameter_pack__new(Dwarf_Die *d
return pack;
}
+/* Returns number of locations found or negative value for errors. */
+static ptrdiff_t __dwarf_getlocations(Dwarf_Attribute *attr,
+ ptrdiff_t offset, Dwarf_Addr *basep,
+ Dwarf_Addr *startp, Dwarf_Addr *endp,
+ Dwarf_Op **expr, size_t *exprlen)
+{
+ int ret;
+
+#if _ELFUTILS_PREREQ(0, 157)
+ ret = dwarf_getlocations(attr, offset, basep, startp, endp, expr, exprlen);
+#else
+ if (offset == 0) {
+ ret = dwarf_getlocation(attr, expr, exprlen);
+ if (ret == 0)
+ ret = 1;
+ }
+#endif
+ return ret;
+}
+
+/* For DW_AT_location 'attr':
+ * - if first location is DW_OP_regXX with expected number, return the register;
+ * otherwise save the register for later return
+ * - if location DW_OP_entry_value(DW_OP_regXX) with expected number is in the
+ * list, return the register; otherwise save register for later return
+ * - otherwise if no register was found for locations, return -1.
+ */
+static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
+{
+ Dwarf_Addr base, start, end;
+ Dwarf_Op *expr, *entry_ops;
+ Dwarf_Attribute entry_attr;
+ size_t exprlen, entry_len;
+ ptrdiff_t offset = 0;
+ int loc_num = -1;
+ int ret = -1;
+
+ while ((offset = __dwarf_getlocations(attr, offset, &base, &start, &end, &expr, &exprlen)) > 0) {
+ loc_num++;
+
+ /* Convert expression list (XX DW_OP_stack_value) -> (XX).
+ * DW_OP_stack_value instructs interpreter to pop current value from
+ * DWARF expression evaluation stack, and thus is not important here.
+ */
+ if (exprlen > 1 && expr[exprlen - 1].atom == DW_OP_stack_value)
+ exprlen--;
+
+ if (exprlen != 1)
+ continue;
+
+ switch (expr->atom) {
+ /* match DW_OP_regXX at first location */
+ case DW_OP_reg0 ... DW_OP_reg31:
+ if (loc_num != 0)
+ break;
+ ret = expr->atom;
+ if (ret == expected_reg)
+ goto out;
+ break;
+ /* match DW_OP_entry_value(DW_OP_regXX) at any location */
+ case DW_OP_entry_value:
+ case DW_OP_GNU_entry_value:
+ if (dwarf_getlocation_attr(attr, expr, &entry_attr) == 0 &&
+ dwarf_getlocation(&entry_attr, &entry_ops, &entry_len) == 0 &&
+ entry_len == 1) {
+ ret = entry_ops->atom;
+ if (ret == expected_reg)
+ goto out;
+ }
For the above if statement, let us do
if (dwarf_getlocation_attr(attr, expr, &entry_attr) == 0 &&
dwarf_getlocation(&entry_attr, &entry_ops, &entry_len) == 0 &&
entry_len == 1 && expected_reg == entry_ops->atom) {
ret = entry_ops->atom;
goto out;
}
This has been discussed in comments of v2:
https://lore.kernel.org/bpf/Zzdh_4Z-e8nl50L6@krava/T/#md3940ef06ee2b2f4bbaf2d1c8dc916f5783330b5
+ break;
+ }
+ }
+out:
+ return ret;
+}
+
static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
struct conf_load *conf, int param_idx)
{
struct parameter *parm = tag__alloc(cu, sizeof(*parm));
if (parm != NULL) {
- Dwarf_Addr base, start, end;
bool has_const_value;
Dwarf_Attribute attr;
- struct location loc;
tag__init(&parm->tag, cu, die);
parm->name = attr_string(die, DW_AT_name, conf);
@@ -1208,35 +1282,21 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
*/
has_const_value = dwarf_attr(die, DW_AT_const_value, &attr) != NULL;
parm->has_loc = dwarf_attr(die, DW_AT_location, &attr) != NULL;
- /* dwarf_getlocations() handles location lists; here we are
- * only interested in the first expr.
- */
- if (parm->has_loc &&
-#if _ELFUTILS_PREREQ(0, 157)
- dwarf_getlocations(&attr, 0, &base, &start, &end,
- &loc.expr, &loc.exprlen) > 0 &&
-#else
- dwarf_getlocation(&attr, &loc.expr, &loc.exprlen) == 0 &&
-#endif
- loc.exprlen != 0) {
+
+ if (parm->has_loc) {
int expected_reg = cu->register_params[param_idx];
- Dwarf_Op *expr = loc.expr;
+ int actual_reg = parameter__reg(&attr, expected_reg);
- switch (expr->atom) {
- case DW_OP_reg0 ... DW_OP_reg31:
+ if (actual_reg < 0)
+ parm->optimized = 1;
+ else if (expected_reg >= 0 && expected_reg != actual_reg)
/* mark parameters that use an unexpected
* register to hold a parameter; these will
* be problematic for users of BTF as they
* violate expectations about register
* contents.
*/
- if (expected_reg >= 0 && expected_reg != expr->atom)
- parm->unexpected_reg = 1;
- break;
- default:
- parm->optimized = 1;
- break;
- }
+ parm->unexpected_reg = 1;
} else if (has_const_value) {
parm->optimized = 1;
}
