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QXpmHandler: fix re-entrancy bug in xpm_color_name

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The xpm_color_name() function returned a pointer to a function-static
buffer. This is infamously non-reentrant, and an actual problem,
because we explicitly allow QImage operations (incl. saving to an
.xpm) from non-GUI-threads.

Fix by using the CSS pattern (Caller-Supplied Storage; also used in
the QAnyStringView(char32_t) and QAnyStringView(QStringBuilder) ctors)
to force the caller to allocate storage in its own stack frame. As a
consequence, we re-gain re-entrancy, but the returned pointer is now
only valid until the end of the full-expression, which isn't a problem,
as all callers immediately pass the result to a consumer (asprintf() or
QByteArray).

[ChangeLog][QtGui][QImage] Fixed a race condition when concurrently
writing .xpm files.

Change-Id: I36d7173d53839a52f5cdf58324474c1b32c71f33
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 73fabadcee71af858388fb245fccf4e96d4ead4e)
This commit is contained in:
Marc Mutz 2021-08-04 10:17:11 +02:00
parent 5fa5013ddf
commit 419505ab01
1 changed files with 12 additions and 3 deletions
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15
src/gui/image/qxpmhandler.cpp
View File

@ -49,6 +49,7 @@
#include <qvariant.h>
#include <algorithm>
#include <array>
QT_BEGIN_NAMESPACE
@ -1055,15 +1056,23 @@ bool qt_read_xpm_image_or_array(QIODevice *device, const char * const * source,
return read_xpm_body(device, source, index, state, cpp, ncols, w, h, image);
}
static const char* xpm_color_name(int cpp, int index)
namespace {
template <size_t N>
struct CharBuffer : std::array<char, N>
{
CharBuffer() {} // avoid value-initializing the whole array
};
}
static const char* xpm_color_name(int cpp, int index, CharBuffer<5> && returnable = {})
{
static char returnable[5];
static const char code[] = ".#abcdefghijklmnopqrstuvwxyzABCD"
"EFGHIJKLMNOPQRSTUVWXYZ0123456789";
// cpp is limited to 4 and index is limited to 64^cpp
if (cpp > 1) {
if (cpp > 2) {
if (cpp > 3) {
returnable[4] = '\0';
returnable[3] = code[index % 64];
index /= 64;
} else
@ -1083,7 +1092,7 @@ static const char* xpm_color_name(int cpp, int index)
returnable[1] = '\0';
returnable[0] = code[index];
return returnable;
return returnable.data();
}