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authorYonghong Song <yhs@fb.com>2022-01-27 07:46:00 -0800
committerAlexei Starovoitov <ast@kernel.org>2022-01-27 12:03:46 -0800
commit7472d5a642c94a0ee1882ff3038de72ffe803a01 (patch)
tree460274714afa46c4f08a2026f0b68c8a7df63cf1 /lib/Kconfig.debug
parent46531a30364bd483bfa1b041c15d42a196e77e93 (diff)
downloadlwn-7472d5a642c94a0ee1882ff3038de72ffe803a01.tar.gz
lwn-7472d5a642c94a0ee1882ff3038de72ffe803a01.zip
compiler_types: define __user as __attribute__((btf_type_tag("user")))
The __user attribute is currently mainly used by sparse for type checking. The attribute indicates whether a memory access is in user memory address space or not. Such information is important during tracing kernel internal functions or data structures as accessing user memory often has different mechanisms compared to accessing kernel memory. For example, the perf-probe needs explicit command line specification to indicate a particular argument or string in user-space memory ([1], [2], [3]). Currently, vmlinux BTF is available in kernel with many distributions. If __user attribute information is available in vmlinux BTF, the explicit user memory access information from users will not be necessary as the kernel can figure it out by itself with vmlinux BTF. Besides the above possible use for perf/probe, another use case is for bpf verifier. Currently, for bpf BPF_PROG_TYPE_TRACING type of bpf programs, users can write direct code like p->m1->m2 and "p" could be a function parameter. Without __user information in BTF, the verifier will assume p->m1 accessing kernel memory and will generate normal loads. Let us say "p" actually tagged with __user in the source code. In such cases, p->m1 is actually accessing user memory and direct load is not right and may produce incorrect result. For such cases, bpf_probe_read_user() will be the correct way to read p->m1. To support encoding __user information in BTF, a new attribute __attribute__((btf_type_tag("<arbitrary_string>"))) is implemented in clang ([4]). For example, if we have #define __user __attribute__((btf_type_tag("user"))) during kernel compilation, the attribute "user" information will be preserved in dwarf. After pahole converting dwarf to BTF, __user information will be available in vmlinux BTF. The following is an example with latest upstream clang (clang14) and pahole 1.23: [$ ~] cat test.c #define __user __attribute__((btf_type_tag("user"))) int foo(int __user *arg) { return *arg; } [$ ~] clang -O2 -g -c test.c [$ ~] pahole -JV test.o ... [1] INT int size=4 nr_bits=32 encoding=SIGNED [2] TYPE_TAG user type_id=1 [3] PTR (anon) type_id=2 [4] FUNC_PROTO (anon) return=1 args=(3 arg) [5] FUNC foo type_id=4 [$ ~] You can see for the function argument "int __user *arg", its type is described as PTR -> TYPE_TAG(user) -> INT The kernel can use this information for bpf verification or other use cases. Current btf_type_tag is only supported in clang (>= clang14) and pahole (>= 1.23). gcc support is also proposed and under development ([5]). [1] http://lkml.kernel.org/r/155789874562.26965.10836126971405890891.stgit@devnote2 [2] http://lkml.kernel.org/r/155789872187.26965.4468456816590888687.stgit@devnote2 [3] http://lkml.kernel.org/r/155789871009.26965.14167558859557329331.stgit@devnote2 [4] https://reviews.llvm.org/D111199 [5] https://lore.kernel.org/bpf/0cbeb2fb-1a18-f690-e360-24b1c90c2a91@fb.com/ Signed-off-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/r/20220127154600.652613-1-yhs@fb.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Diffstat (limited to 'lib/Kconfig.debug')
-rw-r--r--lib/Kconfig.debug8
1 files changed, 8 insertions, 0 deletions
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 14b89aa37c5c..6159859769fa 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -325,6 +325,14 @@ config DEBUG_INFO_BTF
config PAHOLE_HAS_SPLIT_BTF
def_bool $(success, test `$(PAHOLE) --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/'` -ge "119")
+config PAHOLE_HAS_BTF_TAG
+ def_bool $(success, test `$(PAHOLE) --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/'` -ge "123")
+ depends on CC_IS_CLANG
+ help
+ Decide whether pahole emits btf_tag attributes (btf_type_tag and
+ btf_decl_tag) or not. Currently only clang compiler implements
+ these attributes, so make the config depend on CC_IS_CLANG.
+
config DEBUG_INFO_BTF_MODULES
def_bool y
depends on DEBUG_INFO_BTF && MODULES && PAHOLE_HAS_SPLIT_BTF