[patch V4 part 1 20/36] vmlinux.lds.h: Create section for protection against instrumentation

Thomas Gleixner
2020-05-05T14:23:45Z

Some code pathes, especially the low level entry code, must be protected
against instrumentation for various reasons:

- Low level entry code can be a fragile beast, especially on x86.

- With NO_HZ_FULL RCU state needs to be established before using it.

Having a dedicated section for such code allows to validate with tooling
that no unsafe functions are invoked.

Add the .noinstr.text section and the noinstr attribute to mark
functions. noinstr implies notrace. Kprobes will gain a section check
later.

Provide also a set of markers: instr_begin()/end()

These are used to mark code inside a noinstr function which calls
into regular instrumentable text section as safe.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V2: Drop noinstr_call_begin()/end()
---
include/asm-generic/sections.h | 3 +++
include/asm-generic/vmlinux.lds.h | 4 ++++
include/linux/compiler.h | 17 +++++++++++++++++
include/linux/compiler_types.h | 4 ++++
scripts/mod/modpost.c | 2 +-
5 files changed, 29 insertions(+), 1 deletion(-)

--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -53,6 +53,9 @@ extern char __ctors_start[], __ctors_end
/* Start and end of .opd section - used for function descriptors. */
extern char __start_opd[], __end_opd[];

+/* Start and end of instrumentation protected text section */
+extern char __noinstr_text_start[], __noinstr_text_end[];
+
extern __visible const void __nosave_begin, __nosave_end;

/* Function descriptor handling (if any). Override in asm/sections.h */
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -551,6 +551,10 @@
#define TEXT_TEXT \
ALIGN_FUNCTION(); \
*(.text.hot TEXT_MAIN .text.fixup .text.unlikely) \
+ ALIGN_FUNCTION(); \
+ __noinstr_text_start = .; \
+ *(.noinstr.text) \
+ __noinstr_text_end = .; \
*(.text..refcount) \
*(.ref.text) \
MEM_KEEP(init.text*) \
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -120,10 +120,27 @@ void ftrace_likely_update(struct ftrace_
/* Annotate a C jump table to allow objtool to follow the code flow */
#define __annotate_jump_table __section(.rodata..c_jump_table)

+/* Begin/end of an instrumentation safe region */
+#define instr_begin() ({ \
+ asm volatile("%c0:\n\t" \
+ ".pushsection .discard.instr_begin\n\t" \
+ ".long %c0b - .\n\t" \
+ ".popsection\n\t" : : "i" (__COUNTER__)); \
+})
+
+#define instr_end() ({ \
+ asm volatile("%c0:\n\t" \
+ ".pushsection .discard.instr_end\n\t" \
+ ".long %c0b - .\n\t" \
+ ".popsection\n\t" : : "i" (__COUNTER__)); \
+})
+
#else
#define annotate_reachable()
#define annotate_unreachable()
#define __annotate_jump_table
+#define instr_begin() do { } while(0)
+#define instr_end() do { } while(0)
#endif

#ifndef ASM_UNREACHABLE
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -118,6 +118,10 @@ struct ftrace_likely_data {
#define notrace __attribute__((__no_instrument_function__))
#endif

+/* Section for code which can't be instrumented at all */
+#define noinstr \
+ noinline notrace __attribute((__section__(".noinstr.text")))
+
/*
* it doesn't make sense on ARM (currently the only user of __naked)
* to trace naked functions because then mcount is called without
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -948,7 +948,7 @@ static void check_section(const char *mo

#define DATA_SECTIONS ".data", ".data.rel"
#define TEXT_SECTIONS ".text", ".text.unlikely", ".sched.text", \
- ".kprobes.text", ".cpuidle.text"
+ ".kprobes.text", ".cpuidle.text", ".noinstr.text"
#define OTHER_TEXT_SECTIONS ".ref.text", ".head.text", ".spinlock.text", \
".fixup", ".entry.text", ".exception.text", ".text.*", \
".coldtext"