Re: [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check with CONFIG_RETHUNK

Peter Zijlstra
2022-09-07T11:44:56Z

On Wed, Sep 07, 2022 at 12:19:06PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 07, 2022 at 06:49:57PM +0900, Masami Hiramatsu wrote:
> > Yeah, this looks good to me. What I just need is to add expanding
> > queue buffer. (can we use xarray for this purpose?)
>
> Yeah, xarray might just work.
>
> I need to go fetch the kids from school, but if I remember I'll modify
> objtool to tell us the max number required here (for any one particular
> build obviously).

quick hack below suggests we need 405 for a x86_64 defconfig+kvm_guest.config vmlinux.o.

$ OBJTOOL_ARGS="--stats" make O=defconfig-build/ -j12 vmlinux.o
...
max_targets: 405 (hidinput_connect)
...

And if you look at the output of:

$ ./scripts/objdump-func defconfig-build/vmlinux.o hidinput_connect

I'm inclined to believe this. That function is crazy. So yeah, we
definitely need something dynamic.

---
tools/objtool/check.c | 12 ++++++++++++
tools/objtool/include/objtool/elf.h | 1 +
2 files changed, 13 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index e55fdf952a3a..897d3b83ab70 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3295,6 +3295,9 @@ static struct instruction *next_insn_to_validate(struct objtool_file *file,
return next_insn_same_sec(file, insn);
}

+static int max_targets = 0;
+static char *max_name = NULL;
+
/*
* Follow the branch starting at the given instruction, and recursively follow
* any other branches (jumps). Meanwhile, track the frame pointer state at
@@ -3312,6 +3315,14 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,

sec = insn->sec;

+ if (!insn->visited && func) {
+ func->targets++;
+ if (func->targets > max_targets) {
+ max_targets = func->targets;
+ max_name = func->name;
+ }
+ }
+
while (1) {
next_insn = next_insn_to_validate(file, insn);

@@ -4305,6 +4316,7 @@ int check(struct objtool_file *file)
printf("nr_cfi: %ld\n", nr_cfi);
printf("nr_cfi_reused: %ld\n", nr_cfi_reused);
printf("nr_cfi_cache: %ld\n", nr_cfi_cache);
+ printf("max_targets: %d (%s)\n", max_targets, max_name);
}

out:
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 16f4067b82ae..a707becdef50 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -61,6 +61,7 @@ struct symbol {
u8 fentry : 1;
u8 profiling_func : 1;
struct list_head pv_target;
+ int targets;
};

struct reloc {