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digitalmars.D - can we un-deprecate .ptr on arrays in safe code? cf issue 18529

reply Timothee Cour <thelastmammoth gmail.com> writes:
see rationale in https://issues.dlang.org/show_bug.cgi?id=18529
Feb 27 2018
next sibling parent Mike Franklin <slavo5150 yahoo.com> writes:
On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour wrote:
 see rationale in https://issues.dlang.org/show_bug.cgi?id=18529
It looks like the actual deprecation was made with this PR: https://github.com/dlang/dmd/pull/5860 Meaning it's been deprecated for more than a year and a half. I think you're going to have to take it up with Walter. Mike
Feb 27 2018
prev sibling next sibling parent reply bauss <jj_1337 live.dk> writes:
On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour wrote:
 see rationale in https://issues.dlang.org/show_bug.cgi?id=18529
All I see is a rationale for how it can't be replaced, but not a rationale for actually doing it. I can't imagine a "safe" situation where you want to do it. Tbh. just put your code in trusted
Feb 27 2018
parent reply Stefan Koch <uplink.coder googlemail.com> writes:
On Tuesday, 27 February 2018 at 09:23:19 UTC, bauss wrote:
 On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour 
 wrote:
 see rationale in https://issues.dlang.org/show_bug.cgi?id=18529
All I see is a rationale for how it can't be replaced, but not a rationale for actually doing it. I can't imagine a "safe" situation where you want to do it. Tbh. just put your code in trusted
Checking if an array is the slice of another.
Feb 27 2018
next sibling parent reply bauss <jj_1337 live.dk> writes:
On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:
 On Tuesday, 27 February 2018 at 09:23:19 UTC, bauss wrote:
 On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour 
 wrote:
 see rationale in 
 https://issues.dlang.org/show_bug.cgi?id=18529
All I see is a rationale for how it can't be replaced, but not a rationale for actually doing it. I can't imagine a "safe" situation where you want to do it. Tbh. just put your code in trusted
Checking if an array is the slice of another.
Like? assert(a.ptr != b.ptr); Which already works in safe.
Feb 27 2018
parent reply Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:
On Tuesday, 27 February 2018 at 09:58:00 UTC, bauss wrote:
 On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:
 On Tuesday, 27 February 2018 at 09:23:19 UTC, bauss wrote:
 On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour 
 wrote:
 see rationale in 
 https://issues.dlang.org/show_bug.cgi?id=18529
All I see is a rationale for how it can't be replaced, but not a rationale for actually doing it. I can't imagine a "safe" situation where you want to do it. Tbh. just put your code in trusted
Checking if an array is the slice of another.
Like? assert(a.ptr != b.ptr); Which already works in safe.
That only checks if the first element is the same. For a full 'is slice' check you'd need something like this: assert(a.ptr <= b.ptr && b.ptr + b.length <= a.ptr + a.length); // Or: auto c = b.ptr - a.ptr; assert(c >= 0 && c + b.length <= a.length); And trust me, the compiler complains about both of these. Possibly rightfully in the first example, but the latter never does anything scary with the given pointers. -- Simen
Feb 27 2018
next sibling parent Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Tuesday, February 27, 2018 11:33:04 Simen Kjrs via Digitalmars-d wrote:
 And trust me, the compiler complains about both of these.
 Possibly rightfully in the first example, but the latter never
 does anything scary with the given pointers.
As I understand it, the way that safety checks generally work is they check whether a particular operation is safe or not. They don't usually care about what is then done with the result. So, if you do something like take the address of something, that's immediately system regardless of what you do with the result. That changes on some level with DIP 1000 and scope, because then it uses scope to ensure that the lifetime of stuff like pointers doesn't exceed the lifetime of what they point to so that it can then know that taking the address is safe, but without DIP 1000, it takes very little for something to become system. e.g. this is compiles with -dip1000 but otherwise doesn't: void main() safe { int i; assert(&i !is null); } Now, the compiler does seem to be a bit smarter with dynamic arrays and ptr given that this compiles without -dip1000 void main() safe { int[] i; assert(i.ptr !is null); } However, this doesn't compile with -dip1000: void main() safe { int[] i; auto j = i.ptr; assert(j !is null); } and not even this compiles with -dip1000: void main() safe { int[] i; scope j = i.ptr; assert(j !is null); } though I'm inclined to think that that's a bug from what I understand of -dip1000. In any case, safety checks tend to be fairly primitive, so once you start mucking around with pointers, it's not hard to write code that gets treated as system because of a single expression in the code that is clearly safe within the context of the function, but the compiler can't see it. And for better or worse, accessing a dynamic array's ptr member is now system, because it's not safe in all circumstances. If the compiler were smarter, then a number of uses of ptr would probably be safe, but its analysis for stuff like that is usually pretty primitive, in part because making it sophisticated requires stuff like code flow analysis, which the compiler doesn't do a lot of, precisely because it is complicated and easy to get wrong. Walter is particularly leery about making it so that stuff is an error or not based on code flow analysis, and safe falls into that camp. Clearly, some of that is going on with DIP 1000, but that seems to be largely by using the type system to solve the problem rather than doing much in the way of code flow analysis. - Jonathan M Davis
Feb 27 2018
prev sibling next sibling parent reply Timothee Cour <thelastmammoth gmail.com> writes:
this would be more bearable if there was a standard  trusted method to
get array `.ptr`, eg:
in `object.d` (so that it's indeed standard)

```
 trusted  nogc pure nothrow
auto pointer(T)(T a){
  return a.ptr;
}
```

again, the deprecation message is misleading because `&a[0]` isn't
equivalent to `a.ptr`
having something like `pointer` (and making deprecation msg use that)
would be a better mitigation



On Tue, Feb 27, 2018 at 3:56 AM, Jonathan M Davis via Digitalmars-d
<digitalmars-d puremagic.com> wrote:
 On Tuesday, February 27, 2018 11:33:04 Simen Kjærås via Digitalmars-d wrote:
 And trust me, the compiler complains about both of these.
 Possibly rightfully in the first example, but the latter never
 does anything scary with the given pointers.
As I understand it, the way that safety checks generally work is they check whether a particular operation is safe or not. They don't usually care about what is then done with the result. So, if you do something like take the address of something, that's immediately system regardless of what you do with the result. That changes on some level with DIP 1000 and scope, because then it uses scope to ensure that the lifetime of stuff like pointers doesn't exceed the lifetime of what they point to so that it can then know that taking the address is safe, but without DIP 1000, it takes very little for something to become system. e.g. this is compiles with -dip1000 but otherwise doesn't: void main() safe { int i; assert(&i !is null); } Now, the compiler does seem to be a bit smarter with dynamic arrays and ptr given that this compiles without -dip1000 void main() safe { int[] i; assert(i.ptr !is null); } However, this doesn't compile with -dip1000: void main() safe { int[] i; auto j = i.ptr; assert(j !is null); } and not even this compiles with -dip1000: void main() safe { int[] i; scope j = i.ptr; assert(j !is null); } though I'm inclined to think that that's a bug from what I understand of -dip1000. In any case, safety checks tend to be fairly primitive, so once you start mucking around with pointers, it's not hard to write code that gets treated as system because of a single expression in the code that is clearly safe within the context of the function, but the compiler can't see it. And for better or worse, accessing a dynamic array's ptr member is now system, because it's not safe in all circumstances. If the compiler were smarter, then a number of uses of ptr would probably be safe, but its analysis for stuff like that is usually pretty primitive, in part because making it sophisticated requires stuff like code flow analysis, which the compiler doesn't do a lot of, precisely because it is complicated and easy to get wrong. Walter is particularly leery about making it so that stuff is an error or not based on code flow analysis, and safe falls into that camp. Clearly, some of that is going on with DIP 1000, but that seems to be largely by using the type system to solve the problem rather than doing much in the way of code flow analysis. - Jonathan M Davis
Feb 27 2018
parent Steven Schveighoffer <schveiguy yahoo.com> writes:
On 2/27/18 7:20 AM, Timothee Cour wrote:
 this would be more bearable if there was a standard  trusted method to
 get array `.ptr`, eg:
 in `object.d` (so that it's indeed standard)
 
 ```
  trusted  nogc pure nothrow
 auto pointer(T)(T a){
    return a.ptr;
 }
 ```
That completely defeats the purpose of the restriction! Of course we aren't going to do that :) This is the standard way (and it's on you to make sure you don't mess it up): auto x = (() trusted => a.ptr)();
 again, the deprecation message is misleading because `&a[0]` isn't
 equivalent to `a.ptr`
It isn't equivalent, but it is a safe mechanism to do it. If you want to do unsafe things, then you need to use trusted escapes, druntime/phobos is not going to help you there. -Steve
Feb 27 2018
prev sibling parent Seb <seb wilzba.ch> writes:
On Tuesday, 27 February 2018 at 11:33:04 UTC, Simen Kjærås wrote:
 That only checks if the first element is the same. For a full 
 'is slice' check you'd need something like this:

     assert(a.ptr <= b.ptr && b.ptr + b.length <= a.ptr + 
 a.length);

 // Or:

     auto c = b.ptr - a.ptr;
     assert(c >= 0 && c + b.length <= a.length);

 And trust me, the compiler complains about both of these. 
 Possibly rightfully in the first example, but the latter never 
 does anything scary with the given pointers.

 --
   Simen
aka isSliceOf -> https://github.com/dlang/phobos/pull/6147
Feb 27 2018
prev sibling parent reply Dukc <ajieskola gmail.com> writes:
I don't think a just iterated array is automatically set to null, 
so taking it's pointer won't hit a memory-proteted area. So 
undeprectating arr.ptr in  safe would break  safety and be a step 
backward.

If this is an issue, one can define a  trusted function which 
takes a starting pointer from array and casts it to size_t before 
returning it so memory corruption cannot happen via it.

On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:
 Checking if an array is the slice of another.
For that there is also std.array.overlap().
Feb 28 2018
parent Cym13 <cpicard openmailbox.org> writes:
On Wednesday, 28 February 2018 at 22:34:07 UTC, Dukc wrote:
 I don't think a just iterated array is automatically set to 
 null, so taking it's pointer won't hit a memory-proteted area. 
 So undeprectating arr.ptr in  safe would break  safety and be a 
 step backward.
If it cannot be proven safe by the compiler but is safe anyway it doesn't break safe, it should just be trusted. That's what trusted is about, not about abusing functions to trick unsafe things into compiling safe-ly.
 If this is an issue, one can define a  trusted function which 
 takes a starting pointer from array and casts it to size_t 
 before returning it so memory corruption cannot happen via it.

 On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:
 Checking if an array is the slice of another.
For that there is also std.array.overlap().
Feb 28 2018
prev sibling parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Tuesday, February 27, 2018 04:20:38 Timothee Cour via Digitalmars-d 
wrote:
 this would be more bearable if there was a standard  trusted method to
 get array `.ptr`, eg:
 in `object.d` (so that it's indeed standard)

 ```
  trusted  nogc pure nothrow
 auto pointer(T)(T a){
   return a.ptr;
 }
 ```
Except that that's really not how trusted is supposed to be used. The programmer needs to verify that the caller is using a.ptr in a manner that is actually safe, because the compiler is not smart enough to determine that for you. Wrapping it in an trusted function means that the caller won't get an error and that the programmer won't necessarily know that they need to verify the calling code. It's the code that's using ptr that needs to be verified, not the actual accessing of ptr. Hiding the access to ptr within an trusted function goes against that entire idea of trusted and makes it easy to use ptr without realizing that you need to be checking the code that's using it, since you just called a wrapper function to silence the compiler instead of listening the compiler and studying the code using ptr to verify its safety.
 again, the deprecation message is misleading because `&a[0]` isn't
 equivalent to `a.ptr`
 having something like `pointer` (and making deprecation msg use that)
 would be a better mitigation
In almost all cases, &a[0] is equivalent to a.ptr except that it does bounds checking, so it's actually safe and thus doesn't need to be manually verified by the programmer, unlike your pointer function suggestion. If you have a use case where you need a.ptr rather than &a[0], then that just means that you're going to have to verify that your code is safe in spite of using an system operation and mark it trusted if it is. Simply treating a.ptr as safe when it isn't or wrapping it in an trusted function just defeats the purpose of the whole safety system. Now, maybe in some simple cases, the compiler can be improved to detect that what you're doing with a.ptr is actually safe and not give an error, but as long as it can't, any use of a.ptr needs to be verified by the programmer. - Jonathan M Davis
Feb 27 2018
next sibling parent reply Atila Neves <atila.neves gmail.com> writes:
On Tuesday, 27 February 2018 at 12:39:04 UTC, Jonathan M Davis 
wrote:
 On Tuesday, February 27, 2018 04:20:38 Timothee Cour via 
 Digitalmars-d wrote:
 [...]
Except that that's really not how trusted is supposed to be used. The programmer needs to verify that the caller is using a.ptr in a manner that is actually safe, because the compiler is not smart enough to determine that for you. Wrapping it in an trusted function means that the caller won't get an error and that the programmer won't necessarily know that they need to verify the calling code. It's the code that's using ptr that needs to be verified, not the actual accessing of ptr. Hiding the access to ptr within an trusted function goes against that entire idea of trusted and makes it easy to use ptr without realizing that you need to be checking the code that's using it, since you just called a wrapper function to silence the compiler instead of listening the compiler and studying the code using ptr to verify its safety.
 [...]
In almost all cases, &a[0] is equivalent to a.ptr except that it does bounds checking, so it's actually safe and thus doesn't need to be manually verified by the programmer, unlike your pointer function suggestion.
There's a common case where it's not equivalent - when the pointer is null. Imagine I have a C function I want to call: extern(C) void fun(int* things); Imagine also that it's ok to call with null. Well, now I can't use a slice to call this and have it be 1) safe and 2) not throw RangeError. I ran into this the other way. Atila
Feb 27 2018
next sibling parent ag0aep6g <anonymous example.com> writes:
On 02/27/2018 06:32 PM, Atila Neves wrote:
 On Tuesday, 27 February 2018 at 12:39:04 UTC, Jonathan M Davis wrote:
[...]
 In almost all cases, &a[0] is equivalent to a.ptr except that it does 
 bounds checking, so it's actually  safe and thus doesn't need to be 
 manually verified by the programmer, unlike your pointer function 
 suggestion.
There's a common case where it's not equivalent - when the pointer is null. Imagine I have a C function I want to call: extern(C) void fun(int* things); Imagine also that it's ok to call with null. Well, now I can't use a slice to call this and have it be 1) safe and 2) not throw RangeError. I ran into this the other way.
As Jonathan says, you have to manually verify that it's safe, because it generally isn't. `arr.ptr` can be invalid but not null, even in safe code. Consider: ---- void fun(int* things) safe { int x = things is null ? 0 : *things; import std.stdio; writeln(x); } void main() { int[] arr; fun(arr.ptr); /* Ok, prints "0". */ arr = [1, 2, 3]; fun(arr.ptr); /* Ok, prints "1".*/ arr = arr[$ .. $]; /* This is safe. */ fun(arr.ptr); /* Not ok, prints garbage. */ } ---- The first two calls are actually safe and can be trusted. The third call is invalid and must not be possible in safe code. Maybe it would be possible require `arr.ptr` to be valid or null in safe code. This would outlaw `arr[$ .. $]` and bounds checking would have to catch it. I don't know if that would be any practical than banning `arr.ptr`.
Feb 27 2018
prev sibling parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 2/27/18 12:32 PM, Atila Neves wrote:

 There's a common case where it's not equivalent - when the pointer is 
 null. Imagine I have a C function I want to call:
 
 extern(C) void fun(int* things);
 
 Imagine also that it's ok to call with null. Well, now I can't use a 
 slice to call this and have it be 1)  safe and 2) not throw RangeError. 
 I ran into this the other way.
fun(x.length ? &x[0] : null); I think even the compiler could elide the bounds check since you already did it, but I'm not sure that actually happens. -Steve
Feb 27 2018
parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 2/27/18 3:00 PM, Steven Schveighoffer wrote:
 On 2/27/18 12:32 PM, Atila Neves wrote:
 
 There's a common case where it's not equivalent - when the pointer is 
 null. Imagine I have a C function I want to call:

 extern(C) void fun(int* things);

 Imagine also that it's ok to call with null. Well, now I can't use a 
 slice to call this and have it be 1)  safe and 2) not throw 
 RangeError. I ran into this the other way.
fun(x.length ? &x[0] : null);
Hm... borrowing from Timothee's suggestion: trusted nogc pure nothrow T* pointer(T)(T[] a){ return a.length > 0 ? a.ptr : null; } This would be fine and safe, but may not be useful for all purposes. However, it would fix your issue. -Steve
Feb 27 2018
parent Timothee Cour <thelastmammoth gmail.com> writes:
 Hm... borrowing from Timothee's suggestion:
 This would be fine and  safe, but may not be useful for all purposes. However,
it would fix your issue.
how about this: https://github.com/dlang/phobos/pull/6231 On Tue, Feb 27, 2018 at 12:09 PM, Steven Schveighoffer via Digitalmars-d <digitalmars-d puremagic.com> wrote:
 On 2/27/18 3:00 PM, Steven Schveighoffer wrote:
 On 2/27/18 12:32 PM, Atila Neves wrote:

 There's a common case where it's not equivalent - when the pointer is
 null. Imagine I have a C function I want to call:

 extern(C) void fun(int* things);

 Imagine also that it's ok to call with null. Well, now I can't use a
 slice to call this and have it be 1)  safe and 2) not throw RangeError. I
 ran into this the other way.
fun(x.length ? &x[0] : null);
Hm... borrowing from Timothee's suggestion: trusted nogc pure nothrow T* pointer(T)(T[] a){ return a.length > 0 ? a.ptr : null; } This would be fine and safe, but may not be useful for all purposes. However, it would fix your issue. -Steve
Feb 27 2018
prev sibling parent Walter Bright <newshound2 digitalmars.com> writes:
On 2/27/2018 4:39 AM, Jonathan M Davis wrote:
 Except that that's really not how  trusted is supposed to be used. The
 programmer needs to verify that the caller is using a.ptr in a manner that
 is actually  safe, because the compiler is not smart enough to determine
 that for you. Wrapping it in an  trusted function means that the caller
 won't get an error and that the programmer won't necessarily know that they
 need to verify the calling code. It's the code that's using ptr that needs
 to be verified, not the actual accessing of ptr.
 
 Hiding the access to ptr within an  trusted function goes against that
 entire idea of  trusted and makes it easy to use ptr without realizing that
 you need to be checking the code that's using it, since you just called a
 wrapper function to silence the compiler instead of listening the compiler
 and studying the code using ptr to verify its  safety.
Yes, this bears repeating.
Feb 27 2018