Re: [PATCH dwarves 3/3] dwarf_loader: Check DW_OP_[GNU_]entry_value for possible parameter matching

Alan Maguire
2024-11-11T15:40:18Z

On 08/11/2024 18:05, Yonghong Song wrote:
> 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 matchs 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 61552 functions
> in BTF and perf_event_read() is included.
>

hi Yonghong,

I'm confused by these numbers. I would have thought your changes would
have led to a net increase of functions encoded in vmlinux BTF since we
are now likely catching more cases where registers are expected. When I
ran your patches against an LLVM-built kernel, that's what I saw; 70
additional functions were recognized as having expected parameters, and
thus were encoded in BTF. In your case it looks like we lost nearly 1000
functions. Any idea what's going on there? If you can share your config,
LLVM version I can dig into this from my side too. Thanks!

Alan

> Reported-by: Song Liu <song@kernel.org>
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---
> dwarf_loader.c | 81 +++++++++++++++++++++++++++++++++++---------------
> 1 file changed, 57 insertions(+), 24 deletions(-)
>
> diff --git a/dwarf_loader.c b/dwarf_loader.c
> index e0b8c11..1fe44bc 100644
> --- a/dwarf_loader.c
> +++ b/dwarf_loader.c
> @@ -1169,34 +1169,67 @@ static bool check_dwarf_locations(Dwarf_Attribute *attr, struct parameter *parm,
> return false;
>
> #if _ELFUTILS_PREREQ(0, 157)
> - /* dwarf_getlocations() handles location lists; here we are
> - * only interested in the first expr.
> - */
> - if (dwarf_getlocations(attr, 0, &base, &start, &end,
> - &loc.expr, &loc.exprlen) > 0 &&
> - loc.exprlen != 0) {
> - expr = loc.expr;
> -
> - switch (expr->atom) {
> - case DW_OP_reg0 ... DW_OP_reg31:
> - /* 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.
> + bool reg_matched = false, reg_unmatched = false, first_expr_reg = false, ret = false;
> + ptrdiff_t offset = 0;
> + int loc_num = -1;
> +
> + while ((offset = dwarf_getlocations(attr, offset, &base, &start, &end, &loc.expr, &loc.exprlen)) > 0 &&
> + loc.exprlen != 0) {
> + ret = true;
> + loc_num++;
> +
> + for (int i = 0; i < loc.exprlen; i++) {
> + Dwarf_Attribute entry_attr;
> + Dwarf_Op *entry_ops;
> + size_t entry_len;
> +
> + expr = &loc.expr[i];
> + switch (expr->atom) {
> + case DW_OP_reg0 ... DW_OP_reg31:
> + /* first location, first expression */
> + if (loc_num == 0 && i == 0) {
> + if (expected_reg >= 0) {
> + if (expected_reg == expr->atom) {
> + reg_matched = true;
> + return true;
> + } else {
> + reg_unmatched = true;
> + }
> + }
> + first_expr_reg = true;
> + }
> + break;
> + /* For the following dwarf entry (arch x86_64) in parameter locations:
> + * DW_OP_GNU_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
> + * RSI register should be available at the entry of the program.
> */
> - if (expected_reg >= 0 && expected_reg != expr->atom)
> - parm->unexpected_reg = 1;
> - break;
> - default:
> - parm->optimized = 1;
> - break;
> + case DW_OP_entry_value:
> + case DW_OP_GNU_entry_value:
> + if (reg_matched)
> + break;
> + if (dwarf_getlocation_attr (attr, expr, &entry_attr) != 0)
> + break;
> + if (dwarf_getlocation (&entry_attr, &entry_ops, &entry_len) != 0)
> + break;
> + if (entry_len != 1)
> + break;
> + if (expected_reg >= 0 && expected_reg == entry_ops->atom) {
> + reg_matched = true;
> + return true;
> + }
> + break;
> + default:
> + break;
> + }
> }
> -
> - return true;
> }
>
> - return false;
> + if (reg_unmatched)
> + parm->unexpected_reg = 1;
> + else if (ret && !first_expr_reg)
> + parm->optimized = 1;
> +
> + return ret;
> #else
> if (dwarf_getlocation(attr, &loc.expr, &loc.exprlen) == 0 &&
> loc.exprlen != 0) {