C#: Rewrite nullness queries #593
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Port many of the nullness test from Java, as well as add new tests.
Port the SSA-based logic from the Java nullness analyses.
For example, in
```
void M(object x)
{
var y = x != null ? "" : null;
if (y != null)
x.ToString();
}
```
the guard `y != null` implies that the guard `x != null` must be true.
For example, in
```
void M(object x)
{
var y = x == null ? 1 : 2;
if (y == 2)
x.ToString();
}
```
the guard `y == 2` implies that the guard `x == null` must be false,
as the assignment of `2` to `y` is unique.
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jf205
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Dec 4, 2018
| @@ -11,6 +11,8 @@ | |||
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| | *@name of query (Query ID)*| *Impact on results* | *How/why the query has changed* | | |||
| | Off-by-one comparison against container length (cs/index-out-of-bounds) | Fewer false positives | Results have been removed when there are additional guards on the index. | | |||
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calumgrant
reviewed
Dec 6, 2018
csharp/ql/src/CSI/NullAlways.qhelp
Outdated
| <overview> | ||
| <p>If a variable is dereferenced, and the variable has a null value on all possible execution paths | ||
| leading to the dereferencing, it is guaranteed to result in a <code>NullReferenceException</code>. | ||
| <p>If a variable is dereferenced, and the variable has a <code>null</code> |
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Might be worth explaining what "dereference" means.
| return !(b1 == b2); | ||
| } | ||
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| public struct CoOrds |
| { | ||
| public int x, y; | ||
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| public CoOrds(int p1, int p2) |
csharp/ql/src/CSI/NullAlways.ql
Outdated
| select access, "Variable $@ is always null here.", var, var.getName() | ||
| from Dereference d, Ssa::SourceVariable v | ||
| where d.isFirstAlwaysNull(v) | ||
| select d, "Variable '$@' is always null here.", v, v.toString() |
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calumgrant
approved these changes
Dec 7, 2018
This was referenced Nov 29, 2023
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This PR rewrites the nullness queries from scratch, very much based on a similar rewrite for Java done by @aschackmull. Most of the work is done in 055351a, and in addition to reviewing commit-by-commit, I suggest reviewing
Nullness.qllon that commit as-is, rather than looking at the diff.The maybe-
nullquery still has some false positives, exemplified via tests ported from Java. My plan is to address those FPs (using control flow graph splitting) in follow-up pull requests.Review: @calumgrant (QL), @jf205 (qhelp + change notes).