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kernel: 6.6: prevent garbage-cleanup of BPF kfuncs

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Backport an upstream Linux fix to prevent BPF kfunc functions from being
removed during linker optimization when building BTF-enabled vmlinux.

The telltale sign of this occurring is build log warnings (e.g. mips64el):

      BTFIDS  vmlinux
    WARN: resolve_btfids: unresolved symbol bpf_verify_pkcs7_signature
    WARN: resolve_btfids: unresolved symbol bpf_lookup_user_key
    WARN: resolve_btfids: unresolved symbol bpf_lookup_system_key
    WARN: resolve_btfids: unresolved symbol bpf_key_put
    WARN: resolve_btfids: unresolved symbol bpf_iter_task_next
    WARN: resolve_btfids: unresolved symbol bpf_iter_css_task_new
    WARN: resolve_btfids: unresolved symbol bpf_get_file_xattr
    WARN: resolve_btfids: unresolved symbol bpf_ct_insert_entry
      NM      System.map
      SORTTAB vmlinux

Link: https://lore.kernel.org/bpf/ZlkoM6%2FPSxVcGM6X@kodidev-ubuntu/
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>
Link: https://github.com/openwrt/openwrt/pull/15697
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
This commit is contained in:
Tony Ambardar 2024-06-10 02:17:40 -07:00 committed by Christian Marangi
parent 546559587f
commit ce51f27fb0
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2 changed files with 123 additions and 0 deletions
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58
target/linux/generic/backport-6.6/065-v6.10-compiler_types.h-Define-__retain.patch Normal file
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@ -0,0 +1,58 @@
From 443df175be581618d6ff781dc3af3aa1a9ba789d Mon Sep 17 00:00:00 2001
From: Tony Ambardar <Tony.Ambardar@gmail.com>
Date: Fri, 31 May 2024 23:55:55 -0700
Subject: [PATCH 1/2] compiler_types.h: Define __retain for
__attribute__((__retain__))
Some code includes the __used macro to prevent functions and data from
being optimized out. This macro implements __attribute__((__used__)), which
operates at the compiler and IR-level, and so still allows a linker to
remove objects intended to be kept.
Compilers supporting __attribute__((__retain__)) can address this gap by
setting the flag SHF_GNU_RETAIN on the section of a function/variable,
indicating to the linker the object should be retained. This attribute is
available since gcc 11, clang 13, and binutils 2.36.
Provide a __retain macro implementing __attribute__((__retain__)), whose
first user will be the '__bpf_kfunc' tag.
Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/
Cc: stable@vger.kernel.org # v6.6+
Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
---
include/linux/compiler_types.h | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -145,6 +145,29 @@ static inline void __chk_io_ptr(const vo
#define __has_builtin(x) (0)
#endif
+/*
+ * Annotating a function/variable with __retain tells the compiler to place
+ * the object in its own section and set the flag SHF_GNU_RETAIN. This flag
+ * instructs the linker to retain the object during garbage-cleanup or LTO
+ * phases.
+ *
+ * Note that the __used macro is also used to prevent functions or data
+ * being optimized out, but operates at the compiler/IR-level and may still
+ * allow unintended removal of objects during linking.
+ *
+ * Optional: only supported since gcc >= 11, clang >= 13
+ *
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-retain-function-attribute
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#retain
+ */
+#if __has_attribute(__retain__) && \
+ (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \
+ defined(CONFIG_LTO_CLANG))
+# define __retain __attribute__((__retain__))
+#else
+# define __retain
+#endif
+
/* Compiler specific macros. */
#ifdef __clang__
#include <linux/compiler-clang.h>

65
target/linux/generic/backport-6.6/066-v6.10-bpf-Harden-__bpf_kfunc-against-linker-removal.patch Normal file
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@ -0,0 +1,65 @@
From ac507ed9882fd91a94657d68fe9ceac04b957103 Mon Sep 17 00:00:00 2001
From: Tony Ambardar <Tony.Ambardar@gmail.com>
Date: Sat, 1 Jun 2024 00:00:21 -0700
Subject: [PATCH 2/2] bpf: Harden __bpf_kfunc tag against linker kfunc removal
BPF kfuncs are often not directly referenced and may be inadvertently
removed by optimization steps during kernel builds, thus the __bpf_kfunc
tag mitigates against this removal by including the __used macro. However,
this macro alone does not prevent removal during linking, and may still
yield build warnings (e.g. on mips64el):
LD vmlinux
BTFIDS vmlinux
WARN: resolve_btfids: unresolved symbol bpf_verify_pkcs7_signature
WARN: resolve_btfids: unresolved symbol bpf_lookup_user_key
WARN: resolve_btfids: unresolved symbol bpf_lookup_system_key
WARN: resolve_btfids: unresolved symbol bpf_key_put
WARN: resolve_btfids: unresolved symbol bpf_iter_task_next
WARN: resolve_btfids: unresolved symbol bpf_iter_css_task_new
WARN: resolve_btfids: unresolved symbol bpf_get_file_xattr
WARN: resolve_btfids: unresolved symbol bpf_ct_insert_entry
WARN: resolve_btfids: unresolved symbol bpf_cgroup_release
WARN: resolve_btfids: unresolved symbol bpf_cgroup_from_id
WARN: resolve_btfids: unresolved symbol bpf_cgroup_acquire
WARN: resolve_btfids: unresolved symbol bpf_arena_free_pages
NM System.map
SORTTAB vmlinux
OBJCOPY vmlinux.32
Update the __bpf_kfunc tag to better guard against linker optimization by
including the new __retain compiler macro, which fixes the warnings above.
Verify the __retain macro with readelf by checking object flags for 'R':
$ readelf -Wa kernel/trace/bpf_trace.o
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
...
[178] .text.bpf_key_put PROGBITS 00000000 6420 0050 00 AXR 0 0 8
...
Key to Flags:
...
R (retain), D (mbind), p (processor specific)
Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202401211357.OCX9yllM-lkp@intel.com/
Fixes: 57e7c169cd6a ("bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs")
Cc: stable@vger.kernel.org # v6.6+
Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
---
include/linux/btf.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -81,7 +81,7 @@
* as to avoid issues such as the compiler inlining or eliding either a static
* kfunc, or a global kfunc in an LTO build.
*/
-#define __bpf_kfunc __used noinline
+#define __bpf_kfunc __used __retain noinline
/*
* Return the name of the passed struct, if exists, or halt the build if for