[bpf-next,v2,3/5] bpf: support bpf_fastcall patterns for kfuncs

Commit Message

Eduard Zingerman Aug. 17, 2024, 1:51 a.m. UTC

Recognize bpf_fastcall patterns around kfunc calls.
For example, suppose bpf_cast_to_kern_ctx() follows bpf_fastcall
contract (which it does), in such a case allow verifier to rewrite BPF
program below:

  r2 = 1;
  *(u64 *)(r10 - 32) = r2;
  call %[bpf_cast_to_kern_ctx];
  r2 = *(u64 *)(r10 - 32);
  r0 = r2;

By removing the spill/fill pair:

  r2 = 1;
  call %[bpf_cast_to_kern_ctx];
  r0 = r2;

Signed-off-by: Eduard Zingerman <[e-mail]>
---
 kernel/bpf/verifier.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

Comments

kernel test robot Aug. 17, 2024, 8:09 p.m. UTC | #1

Hi Eduard,

kernel test robot noticed the following build warnings:

[auto build test WARNING on bpf-next/master]

url:    https://github.com/intel-lab-lkp/linux/commits/Eduard-Zingerman/bpf-rename-nocsr-bpf_fastcall-in-verifier/20240817-095340
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
patch link:    https://lore.kernel.org/r/20240817015140.1039351-4-eddyz87%40gmail.com
patch subject: [PATCH bpf-next v2 3/5] bpf: support bpf_fastcall patterns for kfuncs
config: arc-randconfig-002-20240817 (https://download.01.org/0day-ci/archive/20240818/[e-mail]/config)
compiler: arceb-elf-gcc (GCC) 13.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240818/[e-mail]/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <[e-mail]>
| Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/

All warnings (new ones prefixed by >>):

   kernel/bpf/verifier.c: In function 'kfunc_fastcall_clobber_mask':
>> kernel/bpf/verifier.c:16146:33: warning: variable 'params' set but not used [-Wunused-but-set-variable]
   16146 |         const struct btf_param *params;
         |                                 ^~~~~~


vim +/params +16146 kernel/bpf/verifier.c

 16142	
 16143	/* Same as helper_fastcall_clobber_mask() but for kfuncs, see comment above */
 16144	static u32 kfunc_fastcall_clobber_mask(struct bpf_kfunc_call_arg_meta *meta)
 16145	{
 16146		const struct btf_param *params;
 16147		u32 vlen, i, mask;
 16148	
 16149		params = btf_params(meta->func_proto);
 16150		vlen = btf_type_vlen(meta->func_proto);
 16151		mask = 0;
 16152		if (!btf_type_is_void(btf_type_by_id(meta->btf, meta->func_proto->type)))
 16153			mask |= BIT(BPF_REG_0);
 16154		for (i = 0; i < vlen; ++i)
 16155			mask |= BIT(BPF_REG_1 + i);
 16156		return mask;
 16157	}
 16158

Eduard Zingerman Aug. 18, 2024, 2:50 a.m. UTC | #2

On Sun, 2024-08-18 at 04:09 +0800, kernel test robot wrote:
> Hi Eduard,
> 
> kernel test robot noticed the following build warnings:
> 
> [auto build test WARNING on bpf-next/master]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/Eduard-Zingerman/bpf-rename-nocsr-bpf_fastcall-in-verifier/20240817-095340
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
> patch link:    https://lore.kernel.org/r/20240817015140.1039351-4-eddyz87%40gmail.com
> patch subject: [PATCH bpf-next v2 3/5] bpf: support bpf_fastcall patterns for kfuncs
> config: arc-randconfig-002-20240817 (https://download.01.org/0day-ci/archive/20240818/[e-mail]/config)
> compiler: arceb-elf-gcc (GCC) 13.2.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240818/[e-mail]/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> > Reported-by: kernel test robot <[e-mail]>
> > Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
> 
> All warnings (new ones prefixed by >>):
> 
>    kernel/bpf/verifier.c: In function 'kfunc_fastcall_clobber_mask':
> > > kernel/bpf/verifier.c:16146:33: warning: variable 'params' set but not used [-Wunused-but-set-variable]
>    16146 |         const struct btf_param *params;
>          |                                 ^~~~~~
> 

Missed this warning.
The params variable is not needed indeed.
Will wait for comments before resubmitting.

[...]

Yonghong Song Aug. 22, 2024, 1:23 a.m. UTC | #3

On 8/16/24 6:51 PM, Eduard Zingerman wrote:
> Recognize bpf_fastcall patterns around kfunc calls.
> For example, suppose bpf_cast_to_kern_ctx() follows bpf_fastcall
> contract (which it does), in such a case allow verifier to rewrite BPF
> program below:
>
>    r2 = 1;
>    *(u64 *)(r10 - 32) = r2;
>    call %[bpf_cast_to_kern_ctx];
>    r2 = *(u64 *)(r10 - 32);
>    r0 = r2;
>
> By removing the spill/fill pair:
>
>    r2 = 1;
>    call %[bpf_cast_to_kern_ctx];
>    r0 = r2;
>
> Signed-off-by: Eduard Zingerman <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b18a21bb5e6e..5dafcfff729e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -16112,7 +16112,7 @@  static int visit_func_call_insn(int t, struct bpf_insn *insns,
  */
 static u32 helper_fastcall_clobber_mask(const struct bpf_func_proto *fn)
 {
-	u8 mask;
+	u32 mask;
 	int i;
 
 	mask = 0;
@@ -16140,6 +16140,28 @@  static bool verifier_inlines_helper_call(struct bpf_verifier_env *env, s32 imm)
 	}
 }
 
+/* Same as helper_fastcall_clobber_mask() but for kfuncs, see comment above */
+static u32 kfunc_fastcall_clobber_mask(struct bpf_kfunc_call_arg_meta *meta)
+{
+	const struct btf_param *params;
+	u32 vlen, i, mask;
+
+	params = btf_params(meta->func_proto);
+	vlen = btf_type_vlen(meta->func_proto);
+	mask = 0;
+	if (!btf_type_is_void(btf_type_by_id(meta->btf, meta->func_proto->type)))
+		mask |= BIT(BPF_REG_0);
+	for (i = 0; i < vlen; ++i)
+		mask |= BIT(BPF_REG_1 + i);
+	return mask;
+}
+
+/* Same as verifier_inlines_helper_call() but for kfuncs, see comment above */
+static bool is_fastcall_kfunc_call(struct bpf_kfunc_call_arg_meta *meta)
+{
+	return false;
+}
+
 /* LLVM define a bpf_fastcall function attribute.
  * This attribute means that function scratches only some of
  * the caller saved registers defined by ABI.
@@ -16237,6 +16259,19 @@  static void mark_fastcall_pattern_for_call(struct bpf_verifier_env *env,
 				  bpf_jit_inlines_helper_call(call->imm));
 	}
 
+	if (bpf_pseudo_kfunc_call(call)) {
+		struct bpf_kfunc_call_arg_meta meta;
+		int err;
+
+		err = fetch_kfunc_meta(env, call, &meta, NULL);
+		if (err < 0)
+			/* error would be reported later */
+			return;
+
+		clobbered_regs_mask = kfunc_fastcall_clobber_mask(&meta);
+		can_be_inlined = is_fastcall_kfunc_call(&meta);
+	}
+
 	if (clobbered_regs_mask == ALL_CALLER_SAVED_REGS)
 		return;