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From 7e5b390a008ccad094a39c350f385d58e8a5102a Mon Sep 17 00:00:00 2001
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From: Karl Williamson <khw@cpan.org>
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Date: Fri, 3 May 2019 13:57:47 -0600
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Subject: [PATCH] Remove undefined behavior from IV shifting
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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It is undefined behavior to shift a negative integer to the left.  This
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commit avoids that by treating the value as unsigned, then casting back
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to integer for return.
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Petr Písař: Ported to 5.30.0 from
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814735a391b874af8f00eaf89469e5ec7f38cd4aa.
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Signed-off-by: Petr Písař <ppisar@redhat.com>
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---
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 asan_ignore |  5 -----
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 pp.c        | 21 ++++++++++++++++++++-
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 2 files changed, 20 insertions(+), 6 deletions(-)
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diff --git a/asan_ignore b/asan_ignore
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index e0f5685..f520546 100644
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--- a/asan_ignore
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+++ b/asan_ignore
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@@ -18,11 +18,6 @@
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 fun:Perl_pp_i_*
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-# Perl's << is defined as using the underlying C's << operator, with the
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-# same undefined behaviour for shifts greater than the word size.
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-# (UVs normally, IVs with 'use integer')
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-
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-fun:Perl_pp_left_shift
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 # this function numifies the field width in eg printf "%10f".
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 # It has its own overflow detection, so don't warn about it
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diff --git a/pp.c b/pp.c
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index 7afb090..3ca04e1 100644
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--- a/pp.c
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+++ b/pp.c
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@@ -1991,10 +1991,29 @@ static IV S_iv_shift(IV iv, int shift, bool left)
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        shift = -shift;
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        left = !left;
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    }
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+
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    if (UNLIKELY(shift >= IV_BITS)) {
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        return iv < 0 && !left ? -1 : 0;
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    }
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-   return left ? iv << shift : iv >> shift;
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+   /* For left shifts, perl 5 has chosen to treat the value as unsigned for
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+    * the * purposes of shifting, then cast back to signed.  This is very
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+    * different from perl 6:
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+    *
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+    * $ perl6 -e 'say -2 +< 5'
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+    * -64
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+    *
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+    * $ ./perl -le 'print -2 << 5'
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+    * 18446744073709551552
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+    * */
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+   if (left) {
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+       if (iv == IV_MIN) { /* Casting this to a UV is undefined behavior */
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+           return 0;
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+       }
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+       return (IV) (((UV) iv) << shift);
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+   }
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+
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+   /* Here is right shift */
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+   return iv >> shift;
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 }
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 #define UV_LEFT_SHIFT(uv, shift) S_uv_shift(uv, shift, TRUE)
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-- 
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2.20.1
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