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finish README and rename int_handler -> exit_and_sleep

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commit ffc44c1360ebb3247dae0742e8feb7a2522fa8ce 1 parent ae9c0d0
@bblum authored
Showing with 168 additions and 14 deletions.
  1. +1 −1  Attributes.hs
  2. +164 −10 README
  3. +1 −1  Rules.hs
  4. +2 −2 aan.h
View
2  Attributes.hs
@@ -23,7 +23,7 @@ stringToRule :: String -> Maybe Rule
stringToRule s =
case s of
"might_sleep" -> Just $ Rule $ Nested 0
- "int_handler" -> Just $ Rule $ Nested 1
+ "nested_one" -> Just $ Rule $ Nested 1
"wont_sleep" -> Just $ Rule Infinity
_ -> Nothing
View
174 README
@@ -112,15 +112,169 @@ An annotation is a pair of a rule and an effect, and is given using the
__attribute__ keyword. The file aan.h defines several commonly-used aliases as
cpp macros.
- MAY_SLEEP - The function might invoke the scheduler, or it might not. It has
- no effect on the context ("Nested 0, IncDec 0").
- WONT_SLEEP - The function is guaranteed not to, and is safe to call from
- atomic context. It has no effect. ("Infinity, IncDec 0")
- INT_HANDLER - The function starts with preemption disabled once, and must
- enable it once (and is allowed to sleep after that). ("Nested
- 1, IncDec -1")
- ENTER_ATOMIC - The function enters an atomic section, without regard for
- nesting. ("
-
+ MAY_SLEEP
+ The function might invoke the scheduler, or it might not. It has no
+ effect on the context ("Nested 0, IncDec 0"). Example: mutex_lock
+ WONT_SLEEP
+ The function is guaranteed not to, and is safe to call from atomic
+ context. It has no effect. ("Infinity, IncDec 0") Example: set_esp0
+ EXIT_AND_SLEEP
+ The function starts with preemption disabled once, and must enable it
+ once (and is allowed to sleep after that). ("Nested 1, IncDec -1")
+ Example: page_fault_handler
+ ENTER_ATOMIC
+ The function disables preemption, without regard for nesting.
+ ("Infinity, Disable") Example: disable_interrupts
+ EXIT_ATOMIC
+ The function enables preemption, without regard for nesting.
+ ("Infinity, Enable") Example: enable_interrupts
+ ENTER_ATOMIC_NESTED
+ The function effects another level of preemption disabling. ("Infinity,
+ IncDec 1") Example: spin_lock
+ EXIT_ATOMIC_NESTED
+ The function reverts a level of preemption disabling. ("Infinity, IncDec
+ -1") Example: spin_unlock
+
+Because C's syntax allows for __attribute__ declarations in many places, there
+is some flexibility with how you annotate functions and function pointers. The
+recommended way, however, is that for functions, the annotation goes before the
+return type (but after storage class specifiers), and for function pointers,
+it goes after the argument list:
+
+ static MAY_SLEEP struct tcb *lookup_by_tid(int tid);
+ typedef void (*swexn_handler_t)(void *arg, ureg_t *ureg) MAY_SLEEP;
+
+
+==== 4. Using the tool ====
+
+For each file you wish to check, run: aan [include paths] path/to/file.c
+
+"list_includes.sh" should be used, from the directory with 410kern in it, to
+generate the include paths.
+
+Annotated versions of 410kern/, with aan.h inside, and spec/, are provided. You
+have to annotate the contents of kern/.
+
+No static analysis is perfect, so even with correct annotations there may be
+either spurious warnings or false positives. Errors are generally correct.
+You will have to manually inspect the code for many cases, to determine either
+that a message is spurious, or that there is a real bug, or that your
+annotations were wrong.
+
+Meaning of messages:
+
+- Informational/tracing messages. These are off by default.
+
+- C typechecking warnings. If the code would confuse GCC, it will confuse AAN.
+
+- AAN warnings. These indicate "dubious" patterns to do with annotations, and
+ mean there might be a bug that we didn't notice.
+ - "merging flow" means the branches of an if/switch ended up with different
+ contexts. AAN chooses the "most restrictive" context at the end.
+ - "backward jump": AAN will not do multi-pass, so something that it said was
+ legal on its first pass might become illegal upon this backwards edge in
+ the control flow graph.
+ - "missing annotation"/"invalid annotation": should be pretty clear.
+ - "variadic args contain arrows": passing function pointers as variadic
+ arguments necessarily loses their type information, like casting to void*.
+
+- Errors. This means either AAN thinks it found an atomic-sleep bug or that your
+ annotation was incorrect.
+ - "unmergeable annotations": in a ternary statement that returns function
+ pointers, the annotations for each are incompatible.
+ - "illegal subtyped function pointer assignment": attempt to assign e.g. a
+ MAY_SLEEP function to a WONT_SLEEP pointer.
+ - "illegal invariant function pointer assignment": in reference cells
+ (mutability), even subtyping is illegal.
+ - "illegal function call": attempt to call e.g. a MAY_SLEEP function in
+ atomic context.
+ - "exit context != advertised effect": The professed effect of the function
+ being checked did not match the changed context that came out at the end.
+ - "not all exit contexts match each other": should be pretty clear.
+ - "illegal context effect": Attempt to decrement the preempt counter when
+ it's already zero.
+
+There are certain idioms and tricks you have to watch out for.
+
+- Casting data to and from void*, or even int, is pretty common. If the thing
+ being casted has function pointers anywhere in it (it is one, it is a struct
+ with one, it is an array of them...), the tool will emit a warning, because
+ this might let the code bypass the soundness of the annotation system. More
+ frequently, this is innocent behaviour, so warnings for void* casts are off by
+ default. You can turn them on with --warn-void-casts, and manually inspect
+ each one, for completeness.
+
+- Some functions are "magic" in terms of their annotation; i.e., they are the
+ functions that implement the rules/effects themselves. It's quite likely the
+ tool will error on these; if I had more time, I would have a "MAGIC"
+ annotation which would make the tool ignore them, but I don't, so I don't.
+
+- Some functions have conditional behaviour. This may require annotating them
+ in a way that will cause errors on their implementation but will accurately
+ reflect their use case (for example, if mutex_unlock checks if the scheduler
+ is locked, and only reschedules if it isn't, mark it WONT_SLEEP rather than
+ MAY_SLEEP). Or, it may require refactoring some of the code itself, to make
+ function boundaries you can actually annotate ("outb", when used to ack the
+ current interrupt, should be EXIT_ATOMIC_NESTED, but otherwise not - consider
+ wrapping that particular use case in a "magic" "acknowledge_interrupt"
+ function).
+
+- Interrupt handlers should in most cases be marked either WONT_SLEEP or
+ EXIT_ATOMIC_NESTED - while rescheduling is sometimes appropriate in them,
+ doing mutex_lock in them is wrong, and that's what you're trying to catch.
+ Even though the context switcher itself is the essence of rescheduling, in
+ some kernels it may be wrong to mark it MAY_SLEEP, e.g. if it has to be called
+ with interrupts already off, if it gets called from the timer handler, etc.
+ It's the functions which indirectly call it (mutex, cond, sem, yield) which
+ you care about.
+
+I advise attempting to annotate each student kernel in two phases: start by
+annotating all functions in all header files, and then pick a .c file and
+annotate the functions in that and check just that file until you are content
+with it.
+
+
+==== 5. Feedback ====
+
+Please note these things, and anything else of interest, while using AAN:
+
+- How long did it take for you to get up to speed on the tool? (How long did it
+ it take you to read this guide, to understand the annotation language and
+ rules, etc.)
+
+- How long did it take you to annotate all the *header files* of the student
+ kernel?
+
+- Once the headers were annotated, how long did it take for you to be "content"
+ with AAN's output on each successive .c file that you tested? (i.e., how much
+ time did you spend manually inspecting, and patching up or ignoring the false
+ positives?)
+
+- How long did it take you to be "content" with AAN's output on every .c file in
+ the kernel?
+
+- What "strange coding idioms" did you encounter, that you either needed a
+ special annotation for, or had to refactor across function boundaries to make
+ sense of?
+
+
+==== 6. Bugs ====
+
+The implementation of AAN is not perfect. Its known deficiencies are:
+
+- memcpy() to copy/paste struct contents around may break the system. This can
+ be caught at least by the void* cast warning.
+
+- We currently assume the body of each loop will run exactly once. A loop body
+ that goes, "mutex_lock(); spin_lock();" and possibly goes around again, is
+ clearly wrong, but will not be caught. The fix for this is trivial, but not
+ implemented.
+
+- It would be nice to have a "MAGIC" annotation that indicates a function is
+ right for sure.
+
+- It does not handle "fall-through" cases in switch statements. (It assumes each
+ case can only be entered from the top of the switch. "case 1: spin_lock();
+ case 2: mutex_lock(); spin_lock(); break;" will foil AAN.)
# vim: tw=80 ts=4 expandtab
View
2  Rules.hs
@@ -20,7 +20,7 @@ instance Ord Context where -- subtyping relation; user-defined
instance Show Rule where -- user-defined
show (Rule (Nested 0)) = "might_sleep"
- show (Rule (Nested 1)) = "int_handler"
+ show (Rule (Nested 1)) = "nested_one"
show (Rule (Nested x)) = "[unknown rule: " ++ show x ++ "]"
show (Rule Infinity) = "wont_sleep"
View
4 aan.h
@@ -11,7 +11,7 @@
/* function annotations */
#define MAY_SLEEP __attribute__((atomic_all_nighters("might_sleep")))
#define WONT_SLEEP __attribute__((atomic_all_nighters("wont_sleep")))
-#define INT_HANDLER __attribute__((atomic_all_nighters("int_handler","exit_nested")))
+#define EXIT_AND_SLEEP __attribute__((atomic_all_nighters("nested_one","exit_nested")))
/* context-changing annotations */
#define ENTER_ATOMIC __attribute__((atomic_all_nighters("wont_sleep","force_disable")))
@@ -22,7 +22,7 @@
#else
#define MAY_SLEEP
#define WONT_SLEEP
-#define INT_HANDLER
+#define EXIT_AND_SLEEP
#define ENTER_ATOMIC
#define EXIT_ATOMIC
#define ENTER_ATOMIC_NESTED
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