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<h3>
Ben Gamari pushed to branch ghc-8.8
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
</h4>
<ul>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/bc05d3599545ddca50dbd8a557fd71785f6cb6fd">bc05d359</a></strong>
<div>
<span>by Ömer Sinan Ağacan</span>
<i>at 2020-05-30T10:20:52-04:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">FastString: fix eager reading of string ptr in hashStr

This read causes NULL dereferencing when len is 0.

Fixes #17909

In the reproducer in #17909 this bug is triggered as follows:

- SimplOpt.dealWithStringLiteral is called with a single-char string
  ("=" in #17909)

- tailFS gets called on the FastString of the single-char string.

- tailFS checks the length of the string, which is 1, and calls
  mkFastStringByteString on the tail of the ByteString, which is an
  empty ByteString as the original ByteString has only one char.

- ByteString's unsafeUseAsCStringLen returns (NULL, 0) for the empty
  ByteString, which is passed to mkFastStringWith.

- mkFastStringWith gets hash of the NULL pointer via hashStr, which
  fails on empty strings because of this bug.

(cherry picked from commit cb1785d9f839e34a3a4892f354f0c51cc6553c0e)
</pre>
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</ul>
<h4>1 changed file:</h4>
<ul>
<li class="file-stats">
<a href="#853e3f0b2adba3af1c88bbccec2b53c4c5532e39">
compiler/utils/FastString.hs
</a>
</li>
</ul>
<h4>Changes:</h4>
<li id="853e3f0b2adba3af1c88bbccec2b53c4c5532e39">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/bc05d3599545ddca50dbd8a557fd71785f6cb6fd#853e3f0b2adba3af1c88bbccec2b53c4c5532e39"><strong>compiler/utils/FastString.hs</strong></a>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -519,16 +519,22 @@ cmpStringPrefix ptr1 ptr2 len =</td>
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<pre style="margin: 0;"> <span id="LC519" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">do</span> <span class="n" style="color: #333;">r</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">memcmp</span> <span class="n" style="color: #333;">ptr1</span> <span class="n" style="color: #333;">ptr2</span> <span class="n" style="color: #333;">len</span></span>
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<pre style="margin: 0;"> <span id="LC520" class="line" lang="haskell">    <span class="n" style="color: #333;">return</span> <span class="p">(</span><span class="n" style="color: #333;">r</span> <span class="o" style="font-weight: 600;">==</span> <span class="mi" style="color: #099;">0</span><span class="p">)</span></span>
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<pre style="margin: 0;"> <span id="LC521" class="line" lang="haskell"></span>
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<pre style="margin: 0;">-<span id="LC522" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC522" class="line" lang="haskell"><span class="n" style="color: #333;">hashStr</span>  <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">Ptr</span> <span class="kt" style="color: #458; font-weight: 600;">Word8</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">Int</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">Int</span></span>
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<pre style="margin: 0;"> <span id="LC523" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- use the Addr to produce a hash value between 0 & m (inclusive)</span></span>
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<pre style="margin: 0;"> <span id="LC524" class="line" lang="haskell"><span class="n" style="color: #333;">hashStr</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">Ptr</span> <span class="n" style="color: #333;">a</span><span class="o" style="font-weight: 600;">#</span><span class="p">)</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">I</span><span class="o" style="font-weight: 600;">#</span> <span class="n" style="color: #333;">len</span><span class="o" style="font-weight: 600;">#</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">loop</span> <span class="mi" style="color: #099;">0</span><span class="o" style="font-weight: 600;">#</span> <span class="mi" style="color: #099;">0</span><span class="o" style="font-weight: 600;">#</span></span>
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<pre style="margin: 0;">-<span id="LC526" class="line" lang="haskell">   <span class="kr" style="font-weight: 600;">where</span></span>
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<pre style="margin: 0;">-<span id="LC527" class="line" lang="haskell">    <span class="n" style="color: #333;">loop</span> <span class="n" style="color: #333;">h</span> <span class="n" style="color: #333;">n</span> <span class="o" style="font-weight: 600;">|</span> <span class="n" style="color: #333;">isTrue</span><span class="o" style="font-weight: 600;">#</span> <span class="p">(</span><span class="n" style="color: #333;">n</span> <span class="o" style="font-weight: 600;">==#</span> <span class="n" style="color: #333;">len</span><span class="o" style="font-weight: 600;">#</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">I</span><span class="o" style="font-weight: 600;">#</span> <span class="n" style="color: #333;">h</span></span>
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<pre style="margin: 0;">-<span id="LC528" class="line" lang="haskell">             <span class="o" style="font-weight: 600;">|</span> <span class="n" style="color: #333;">otherwise</span>  <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">loop</span> <span class="n" style="color: #333;">h2</span> <span class="p">(</span><span class="n" style="color: #333;">n</span> <span class="o" style="font-weight: 600;">+#</span> <span class="mi" style="color: #099;">1</span><span class="o" style="font-weight: 600;">#</span><span class="p">)</span></span>
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<pre style="margin: 0;">-<span id="LC529" class="line" lang="haskell">          <span class="kr" style="font-weight: 600;">where</span></span>
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<pre style="margin: 0;">-<span id="LC530" class="line" lang="haskell">            <span class="o" style="font-weight: 600;">!</span><span class="n" style="color: #333;">c</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">ord</span><span class="o" style="font-weight: 600;">#</span> <span class="p">(</span><span class="n" style="color: #333;">indexCharOffAddr</span><span class="o" style="font-weight: 600;">#</span> <span class="n" style="color: #333;">a</span><span class="o" style="font-weight: 600;">#</span> <span class="n" style="color: #333;">n</span><span class="p">)</span></span>
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<pre style="margin: 0;">-<span id="LC531" class="line" lang="haskell">            <span class="o" style="font-weight: 600;">!</span><span class="n" style="color: #333;">h2</span> <span class="o" style="font-weight: 600;">=</span> <span class="p">(</span><span class="n" style="color: #333;">h</span> <span class="o" style="font-weight: 600;">*#</span> <span class="mi" style="color: #099;">16777619</span><span class="o" style="font-weight: 600;">#</span><span class="p">)</span> <span class="p">`</span><span class="n" style="color: #333;">xorI</span><span class="o" style="font-weight: 600;">#</span><span class="p">`</span> <span class="n" style="color: #333;">c</span></span>
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<td class="diff-line-num new new_line" data-linenumber="525" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
525
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<pre style="margin: 0;">+<span id="LC525" class="line" lang="haskell">  <span class="kr" style="font-weight: 600;">where</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="526" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
526
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<pre style="margin: 0;">+<span id="LC526" class="line" lang="haskell">    <span class="n" style="color: #333;">loop</span> <span class="n" style="color: #333;">h</span> <span class="n" style="color: #333;">n</span> <span class="o" style="font-weight: 600;">=</span></span>
</pre>
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</td>
<td class="diff-line-num new new_line" data-linenumber="527" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
527
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<pre style="margin: 0;">+<span id="LC527" class="line" lang="haskell">      <span class="kr" style="font-weight: 600;">if</span> <span class="n" style="color: #333;">isTrue</span><span class="o" style="font-weight: 600;">#</span> <span class="p">(</span><span class="n" style="color: #333;">n</span> <span class="o" style="font-weight: 600;">==#</span> <span class="n" style="color: #333;">len</span><span class="o" style="font-weight: 600;">#</span><span class="p">)</span> <span class="kr" style="font-weight: 600;">then</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="528" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
528
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<pre style="margin: 0;">+<span id="LC528" class="line" lang="haskell">        <span class="kt" style="color: #458; font-weight: 600;">I</span><span class="o" style="font-weight: 600;">#</span> <span class="n" style="color: #333;">h</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="529" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
529
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<pre style="margin: 0;">+<span id="LC529" class="line" lang="haskell">      <span class="kr" style="font-weight: 600;">else</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="530" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
530
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<pre style="margin: 0;">+<span id="LC530" class="line" lang="haskell">        <span class="kr" style="font-weight: 600;">let</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="531" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
531
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<pre style="margin: 0;">+<span id="LC531" class="line" lang="haskell">          <span class="c1" style="color: #998; font-style: italic;">-- DO NOT move this let binding! indexCharOffAddr# reads from the</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="532" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
532
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<pre style="margin: 0;">+<span id="LC532" class="line" lang="haskell">          <span class="c1" style="color: #998; font-style: italic;">-- pointer so we need to evaluate this based on the length check</span></span>
</pre>
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533
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<pre style="margin: 0;">+<span id="LC533" class="line" lang="haskell">          <span class="c1" style="color: #998; font-style: italic;">-- above. Not doing this right caused #17909.</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="534" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
534
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<pre style="margin: 0;">+<span id="LC534" class="line" lang="haskell">          <span class="o" style="font-weight: 600;">!</span><span class="n" style="color: #333;">c</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">ord</span><span class="o" style="font-weight: 600;">#</span> <span class="p">(</span><span class="n" style="color: #333;">indexCharOffAddr</span><span class="o" style="font-weight: 600;">#</span> <span class="n" style="color: #333;">a</span><span class="o" style="font-weight: 600;">#</span> <span class="n" style="color: #333;">n</span><span class="p">)</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="535" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
535
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<pre style="margin: 0;">+<span id="LC535" class="line" lang="haskell">          <span class="o" style="font-weight: 600;">!</span><span class="n" style="color: #333;">h2</span> <span class="o" style="font-weight: 600;">=</span> <span class="p">(</span><span class="n" style="color: #333;">h</span> <span class="o" style="font-weight: 600;">*#</span> <span class="mi" style="color: #099;">16777619</span><span class="o" style="font-weight: 600;">#</span><span class="p">)</span> <span class="p">`</span><span class="n" style="color: #333;">xorI</span><span class="o" style="font-weight: 600;">#</span><span class="p">`</span> <span class="n" style="color: #333;">c</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="536" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
536
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<pre style="margin: 0;">+<span id="LC536" class="line" lang="haskell">        <span class="kr" style="font-weight: 600;">in</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="537" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
537
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<pre style="margin: 0;">+<span id="LC537" class="line" lang="haskell">          <span class="n" style="color: #333;">loop</span> <span class="n" style="color: #333;">h2</span> <span class="p">(</span><span class="n" style="color: #333;">n</span> <span class="o" style="font-weight: 600;">+#</span> <span class="mi" style="color: #099;">1</span><span class="o" style="font-weight: 600;">#</span><span class="p">)</span></span>
</pre>
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532
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<td class="diff-line-num new_line" data-linenumber="538" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">
538
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<pre style="margin: 0;"> <span id="LC538" class="line" lang="haskell"></span>
</pre>
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<td class="diff-line-num old_line" data-linenumber="533" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">
533
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<td class="diff-line-num new_line" data-linenumber="539" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">
539
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<pre style="margin: 0;"> <span id="LC539" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- -----------------------------------------------------------------------------</span></span>
</pre>
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534
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<td class="diff-line-num new_line" data-linenumber="540" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">
540
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<pre style="margin: 0;"> <span id="LC540" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Operations</span></span>
</pre>
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