Re: [PATCH v4 0/4]New attribute "counted_by" to annotate bounds for C99 FAM(PR108896)

[Qing Zhao <qing.zhao@oracle.com>]

Okay, Based on the comments so far, I will work on the 5th version of the patch, major changes will include:

1. Change the return type of the routine .ACCESS_WITH_SIZE 
    FROM:
      Pointer to the type of the element of the flexible array;
    TO:
       Pointer to the type of the flexible array;

     And then wrap the call with an indirection reference. 

2. Adjust all other parts with this change, (this will simplify the bound sanitizer instrument);

3. Add the fixes to the kernel building failures, which include:

     A. The operator “typeof” cannot return correct type for a->array; 
	(I guess that the above change 1 might automatically resolve this issue)
     B. The operator “&” cannot return correct address for a->array;

4. Correctly handle the case when the value of “counted-by” is zero or negative as following

    4.1 . Update the counted-by doc with the following:

     When the counted-by field is assigned a negative integer value, the compiler will treat the value as zero. 

    4.2.   (possibly) Adjust __bdos and array bound sanitizer to handle correctly when “counted-by” is zero. 

           __bdos will return size 0 when counted-by is zero;

          Array bound sanitizer will report out-of-bound when the counted-by is zero for any array access. 

Let me know if I missed anything.

Thanks a lot for all the comments

 Qing
    

> On Jan 23, 2024, at 7:29 PM, Qing Zhao <qing.zhao@oracle.com> wrote:
> 
> Hi,
> 
> This is the 4th version of the patch.
> 
> It based on the following proposal:
> 
> https://gcc.gnu.org/pipermail/gcc-patches/2023-November/635884.html
> Represent the missing dependence for the "counted_by" attribute and its consumers
> 
> ******The summary of the proposal is:
> 
> * Add a new internal function ".ACCESS_WITH_SIZE" to carry the size information for every reference to a FAM field;
> * In C FE, Replace every reference to a FAM field whose TYPE has the "counted_by" attribute with the new internal function ".ACCESS_WITH_SIZE";
> * In every consumer of the size information, for example, BDOS or array bound sanitizer, query the size information or ACCESS_MODE information from the new internal function;
> * When expansing to RTL, replace the internal function with the actual reference to the FAM field;
> * Some adjustment to ipa alias analysis, and other SSA passes to mitigate the impact to the optimizer and code generation.
> 
> 
> ******The new internal function
> 
>  .ACCESS_WITH_SIZE (REF_TO_OBJ, REF_TO_SIZE, CLASS_OF_SIZE, SIZE_OF_SIZE, ACCESS_MODE, INDEX)
> 
> INTERNAL_FN (ACCESS_WITH_SIZE, ECF_LEAF | ECF_NOTHROW, NULL)
> 
> which returns the "REF_TO_OBJ" same as the 1st argument;
> 
> Both the return type and the type of the first argument of this function have been converted from the incomplete array type to the corresponding pointer type.
> 
> Please see the following link for why:
> https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638793.html
> https://gcc.gnu.org/pipermail/gcc-patches/2023-December/639605.html
> 
> 1st argument "REF_TO_OBJ": The reference to the object;
> 2nd argument "REF_TO_SIZE": The reference to the size of the object,
> 3rd argument "CLASS_OF_SIZE": The size referenced by the REF_TO_SIZE represents
>   0: unknown;
>   1: the number of the elements of the object type;
>   2: the number of bytes;
> 4th argument "PRECISION_OF_SIZE": The precision of the integer that REF_TO_SIZE points;
> 5th argument "ACCESS_MODE":
>  -1: Unknown access semantics
>   0: none
>   1: read_only
>   2: write_only
>   3: read_write
> 6th argument "INDEX": the INDEX for the original array reference.
>  -1: Unknown
> 
> NOTE: The 6th Argument is added for bound sanitizer instrumentation.
> 
> ****** The Patch sets included:
> 
> 1. Provide counted_by attribute to flexible array member field;
>      which includes:
>      * "counted_by" attribute documentation;
>      * C FE handling of the new attribute;
>        syntax checking, error reporting;
>      * testing cases;
> 
> 2. Convert "counted_by" attribute to/from .ACCESS_WITH_SIZE.
>      which includes:
>      * The definition of the new internal function .ACCESS_WITH_SIZE in internal-fn.def.
>      * C FE converts every reference to a FAM with "counted_by" attribute to a call to the internal function .ACCESS_WITH_SIZE.
>        (build_component_ref in c_typeck.cc)
>        This includes the case when the object is statically allocated and initialized.
>        In order to make this working, we should update initializer_constant_valid_p_1 and output_constant in varasm.cc to include calls to .ACCESS_WITH_SIZE.
> 
>        However, for the reference inside "offsetof", ignore the "counted_by" attribute since it's not useful at all. (c_parser_postfix_expression in c/c-parser.cc)
> 
>      * Convert every call to .ACCESS_WITH_SIZE to its first argument.
>        (expand_ACCESS_WITH_SIZE in internal-fn.cc)
>      * adjust alias analysis to exclude the new internal from clobbering anything.
>        (ref_maybe_used_by_call_p_1 and call_may_clobber_ref_p_1 in tree-ssa-alias.cc)
>      * adjust dead code elimination to eliminate the call to .ACCESS_WITH_SIZE when
>        it's LHS is eliminated as dead code.
>        (eliminate_unnecessary_stmts in tree-ssa-dce.cc)
>      * Provide the utility routines to check the call is .ACCESS_WITH_SIZE and
>        get the reference from the call to .ACCESS_WITH_SIZE.
>        (is_access_with_size_p and get_ref_from_access_with_size in tree.cc)
>      * testing cases. (for offsetof, static initialization, generation of calls to
>        .ACCESS_WITH_SIZE, code runs correctly after calls to .ACCESS_WITH_SIZE are
>        converted back)
> 
> 3. Use the .ACCESS_WITH_SIZE in builtin object size (sub-object only)
>      which includes:
>      * use the size info of the .ACCESS_WITH_SIZE for sub-object.
>      * testing cases. 
> 
> 4 Use the .ACCESS_WITH_SIZE in bound sanitizer
>      Since the result type of the call to .ACCESS_WITH_SIZE is a pointer to
>        the element type. The original array_ref is converted to an indirect_ref,
>        which introduced two issues for the instrumenation of bound sanitizer:
> 
>        A. The index for the original array_ref was mixed into the offset
>        expression for the new indirect_ref.
> 
>        In order to make the instrumentation for the bound sanitizer easier, one
>        more argument for the function .ACCESS_WITH_SIZE is added to record this
>        original index for the array_ref. then later during bound instrumentation,
>        get the index from the additional argument from the call to the function
>        .ACCESS_WITH_SIZE.
> 
>        B. In the current bound sanitizer, no instrumentation will be inserted for
>        an indirect_ref.
> 
>        In order to add instrumentation for an indirect_ref with a call to
>        .ACCESS_WITH_SIZE, we should specially handle the indirect_ref with a
>        call to .ACCESS_WITH_SIZE, and get the index and bound info from the
>        arguments of the call.
> 
>      which includes:
>      * Record the index to the 6th argument of the call to .ACCESS_WITH_SIZE.
>      * Instrument indirect_ref with call to .ACCESS_WITH_SIZE for bound sanitizer.
>      * testing cases. (additional testing cases for dynamic array support)
> 
> ******Remaining works: 
> 
> 5  Improve __bdos to use the counted_by info in whole-object size for the structure with FAM.
> 6  Emit warnings when the user breaks the requirments for the new counted_by attribute
>   compilation time: -Wcounted-by
>   run time: -fsanitizer=counted-by
>      * The initialization to the size field should be done before the first reference to the FAM field.
>      * the array has at least # of elements specified by the size field all the time during the program.
> 
> I have bootstrapped and regression tested on both x86 and aarch64, no issue.
> 
> Let me know your comments.
> 
> thanks.
> 
> Qing