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    SubjectRe: [RESEND][patch V3 03/23] vmlinux.lds.h: Create section for protection against instrumentation
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    On 3/20/20 6:59 PM, Thomas Gleixner wrote:
    > 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 two sets of markers:
    >
    > - instr_begin()/end()
    >
    > This is used to mark code inside in a noinstr function which calls
    > into regular instrumentable text section as safe.
    >
    > - noinstr_call_begin()/end()
    >
    > Same as above but used to mark indirect calls which cannot be tracked by
    > tooling and need to be audited manually.
    >
    > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    > ---
    > include/asm-generic/sections.h | 3 +++
    > include/asm-generic/vmlinux.lds.h | 4 ++++
    > include/linux/compiler.h | 24 ++++++++++++++++++++++++
    > include/linux/compiler_types.h | 4 ++++
    > scripts/mod/modpost.c | 2 +-
    > 5 files changed, 36 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
    > @@ -550,6 +550,10 @@
    > #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
    > @@ -120,12 +120,36 @@ 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
    >
    > +/*
    > + * Annotation for audited indirect calls. Distinct from instr_begin() on
    > + * purpose because the called function has to be noinstr as well.
    > + */
    > +#define noinstr_call_begin() instr_begin()
    > +#define noinstr_call_end() instr_end()
    > +

    Do you have an example of noinstr_call_begin()/end() usage? I have some
    hard time figuring out why we need to call them out.

    Thanks,

    alex.


    > #ifndef ASM_UNREACHABLE
    > # define ASM_UNREACHABLE
    > #endif
    > --- 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
    > @@ -953,7 +953,7 @@ 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"
    >

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