> On Oct 5, 2023, at 4:08 PM, Siddhesh Poyarekar <[email protected]> wrote:
>
> On 2023-08-25 11:24, Qing Zhao wrote:
>> This is the 3rd version of the patch, per our discussion based on the
>> review comments for the 1st and 2nd version, the major changes in this
>> version are:
>
> Hi Qing,
>
> I hope the review was helpful. Overall, a couple of things to consider:
>
> 1. How would you handle potential reordering between assignment of the size
> to the counted_by field with the __bdos call that may consume it? You'll
> probably need to express some kind of dependency there or in the worst case,
> insert a barrier to disallow reordering.
Good point!
So, your example in the respond to [V3][PATCH 2/3]Use the counted_by atribute
info in builtin object size [PR108896]:
“
Maybe another test where the allocation, size assignment and __bdos call happen
in the same function, where the allocator is not recognized by gcc:
void *
__attribute__ ((noinline))
alloc (size_t sz)
{
return __builtin_malloc (sz);
}
void test (size_t sz)
{
array_annotated = alloc (sz);
array_annotated->b = sz;
return __builtin_dynamic_object_size (array_annotated->c, 1);
}
The interesting thing to test (and ensure in the codegen) is that the
assignment to array_annotated->b does not get reordered to below the
__builtin_dynamic_object_size call since technically there is no data
dependency between the two.
“
Will test on this.
Not sure whether the current GCC alias analysis is able to distinguish one
field of a structure from another field of the same structure, if YES, then
We need to add an explicit dependency edge from the write to
“array_annotated->b” to the call to
“__builtin_dynamic_object_size(array_annotated->c,1)”.
I will check on this and see how to resolve this issue.
I guess the possible solution is that we can add an implicit ref to
“array_annotated->b” at the call to
“__builtin_dynamic_object_size(array_annotated->c, 1)” if the counted_by
attribute is available. That should resolve the issue.
Richard, what do you think on this?
>
> 2. How would you handle signedness of the size field? The size gets
> converted to sizetype everywhere it is used and overflows/underflows may
> produce interesting results. Do you want to limit the types to unsigned or
> do you want to add a disclaimer in the docs? The former seems like the
> *right* thing to do given that it is a new feature; best to enforce the
> cleaner habit at the outset.
As I replied to Martin in another email, I plan to do the following to resolve
this issue:
1. No specification for signed or unsigned for counted_by field.
2. Add a sanitizer option -fsanitize=counted-by-bound to catch the cases when
the size of the counted-by is not positive.
Then, we will be consistent with the handling of VLA.
So, I will not change anything for the current patch.
However, I will add the sanitizer option in a followup patch set.
Let me know your opinion.
thanks.
Qing
>
> Thanks,
> Sid
>
>> ***Against 1st version:
>> 1. change the name "element_count" to "counted_by";
>> 2. change the parameter for the attribute from a STRING to an
>> Identifier;
>> 3. Add logic and testing cases to handle anonymous structure/unions;
>> 4. Clarify documentation to permit the situation when the allocation
>> size is larger than what's specified by "counted_by", at the same time,
>> it's user's error if allocation size is smaller than what's specified by
>> "counted_by";
>> 5. Add a complete testing case for using counted_by attribute in
>> __builtin_dynamic_object_size when there is mismatch between the
>> allocation size and the value of "counted_by", the expecting behavior
>> for each case and the explanation on why in the comments.
>> ***Against 2rd version:
>> 1. Identify a tree node sharing issue and fixed it in the routine
>> "component_ref_get_counted_ty" of tree.cc;
>> 2. Update the documentation and testing cases with the clear usage
>> of the fomula to compute the allocation size:
>> MAX (sizeof (struct A), offsetof (struct A, array[0]) + counted_by *
>> sizeof(element))
>> (the algorithm used in tree-object-size.cc is correct).
>> In this set of patches, the major functionality provided is:
>> 1. a new attribute "counted_by";
>> 2. use this new attribute in bound sanitizer;
>> 3. use this new attribute in dynamic object size for subobject size;
>> As discussed, I plan to add two more separate patches sets after this initial
>> patch set is approved and committed.
>> set 1. A new warning option and a new sanitizer option for the user error
>> when the allocation size is smaller than the value of "counted_by".
>> set 2. An improvement to __builtin_dynamic_object_size for whole-object
>> size of the structure with FAM annaoted with counted_by.
>> there are also some existing bugs in tree-object-size.cc identified
>> during the study, and PRs were filed to record them. these bugs will
>> be fixed seperately with individual patches:
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111030
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111040
>> Bootstrapped and regression tested on both aarch64 and X86, no issue.
>> Please see more details on the description of this work on:
>> https://gcc.gnu.org/pipermail/gcc-patches/2023-May/619708.html
>> and more discussions on
>> https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626376.html
>> Okay for committing?
>> thanks.
>> Qing
>> Qing Zhao (3):
>> Provide counted_by attribute to flexible array member field (PR108896)
>> Use the counted_by atribute info in builtin object size [PR108896]
>> Use the counted_by attribute information in bound sanitizer[PR108896]
>> gcc/c-family/c-attribs.cc | 54 ++++-
>> gcc/c-family/c-common.cc | 13 ++
>> gcc/c-family/c-common.h | 1 +
>> gcc/c-family/c-ubsan.cc | 16 ++
>> gcc/c/c-decl.cc | 79 +++++--
>> gcc/doc/extend.texi | 77 +++++++
>> .../gcc.dg/flex-array-counted-by-2.c | 74 ++++++
>> .../gcc.dg/flex-array-counted-by-3.c | 210 ++++++++++++++++++
>> gcc/testsuite/gcc.dg/flex-array-counted-by.c | 40 ++++
>> .../ubsan/flex-array-counted-by-bounds-2.c | 27 +++
>> .../ubsan/flex-array-counted-by-bounds.c | 46 ++++
>> gcc/tree-object-size.cc | 37 ++-
>> gcc/tree.cc | 133 +++++++++++
>> gcc/tree.h | 15 ++
>> 14 files changed, 797 insertions(+), 25 deletions(-)
>> create mode 100644 gcc/testsuite/gcc.dg/flex-array-counted-by-2.c
>> create mode 100644 gcc/testsuite/gcc.dg/flex-array-counted-by-3.c
>> create mode 100644 gcc/testsuite/gcc.dg/flex-array-counted-by.c
>> create mode 100644
>> gcc/testsuite/gcc.dg/ubsan/flex-array-counted-by-bounds-2.c
>> create mode 100644 gcc/testsuite/gcc.dg/ubsan/flex-array-counted-by-bounds.c
