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Avoid referencing uninitialized memory in PerlIOScalar_write #24063

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Avoid referencing uninitialized memory in PerlIOScalar_write #24063

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3ea1aa9
PerlIOScalar_write: initialize target scalar if undefined
t-a-k Jan 7, 2026
863c99c
PerlIOScalar_write: avoid SvCUR() on an undefined scalar
t-a-k Jan 17, 2026
47026e3
t/io/scalar.t: Changed BAIL_OUT to die
t-a-k Jan 17, 2026
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6 changes: 3 additions & 3 deletions perlio.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1265,14 +1265,14 @@ PerlIOScalar_write(pTHX_ PerlIO * f, const void *vbuf, Size_t count)
SETERRNO(EINVAL, SS_IVCHAN);
return 0;
}
/* Avoid calling SvCUR() on undef'ed SVs */
STRLEN const cur = SvOK(sv) ? SvCUR(sv) : 0;
if ((PerlIOBase(f)->flags) & PERLIO_F_APPEND) {
dst = SvGROW(sv, SvCUR(sv) + count + 1);
dst = SvGROW(sv, cur + count + 1);
offset = SvCUR(sv);
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@richardleach richardleach Jan 17, 2026

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Could this now be offset = cur; ?

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@t-a-k t-a-k Jan 18, 2026

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Could this now be offset = cur; ?

I think this has its pros and cons. Referencing cur after SvGROW will make this variable live across the function call, which will impose more register pressure and might worsen performance. (I assume that SvCUR is always valid after SvGROW, and re-fetching SvCUR is not so complex operation.)

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@tonycoz tonycoz Jan 19, 2026

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You could move the assignment before the SvGROW() to avoid that.

Though I doubt the extra memory access of the extra SvCUR() will make any measurable difference.

s->posn = offset + count;
}
else {
STRLEN const cur = SvCUR(sv);

/* ensure we don't try to create ridiculously large
* SVs on small platforms
*/
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6 changes: 6 additions & 0 deletions pod/perldelta.pod
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Expand Up @@ -374,6 +374,12 @@ used to be parsed incorrectly.

=item *

When a scalar variable used as the target of a filehandle is C<undef>'ed
while being output, garbage bytes would be left in that scalar.
[L<GH #24008|https://github.com/Perl/perl5/issues/24008>]

=item *

XXX

=back
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10 changes: 9 additions & 1 deletion t/io/scalar.t
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Expand Up @@ -12,7 +12,7 @@ use strict;
use Fcntl qw(SEEK_SET SEEK_CUR SEEK_END); # Not 0, 1, 2 everywhere.
use Errno qw(EACCES);

plan(128);
plan(129);

my $fh;
my $var = "aaa\n";
Expand Down Expand Up @@ -528,3 +528,11 @@ SKIP:
select((select($fh), ++$|)[0]);
ok(!(print $fh "x"), "write to a large offset");
}

{ # GH #24008
open my $fh, '>', \my $str or die $!;
print $fh "xxxxx";
undef $str;
print $fh "y";
is($str, "\0\0\0\0\0y", "write to undef'ed variable");
}
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