Re: [PATCH v3 02/28] glib-compat: Introduce g_memdup2() wrapper

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 12/16/21 15:11, Alex Bennée wrote:
> Philippe Mathieu-Daudé <philmd@redhat.com> writes:
> 
>> When experimenting raising GLIB_VERSION_MIN_REQUIRED to 2.68
>> (Fedora 34 provides GLib 2.68.1) we get:
>>
>>   hw/virtio/virtio-crypto.c:245:24: error: 'g_memdup' is deprecated: Use 'g_memdup2' instead [-Werror,-Wdeprecated-declarations]
>>   ...
>>
>> g_memdup() has been updated by g_memdup2() to fix eventual security
>> issues (size argument is 32-bit and could be truncated / wrapping).
>> GLib recommends to copy their static inline version of g_memdup2():
>> https://discourse.gnome.org/t/port-your-module-from-g-memdup-to-g-memdup2-now/5538
>>
>> Our glib-compat.h provides a comment explaining how to deal with
>> these deprecated declarations (see commit e71e8cc0355
>> "glib: enforce the minimum required version and warn about old APIs").
>>
>> Following this comment suggestion, implement the g_memdup2_qemu()
>> wrapper to g_memdup2(), and use the safer equivalent inlined when
>> we are using pre-2.68 GLib.
>>
>> Reported-by: Eric Blake <eblake@redhat.com>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>  include/glib-compat.h | 37 +++++++++++++++++++++++++++++++++++++
>>  1 file changed, 37 insertions(+)
>>
>> diff --git a/include/glib-compat.h b/include/glib-compat.h
>> index 9e95c888f54..8d01a8c01fb 100644
>> --- a/include/glib-compat.h
>> +++ b/include/glib-compat.h
>> @@ -68,6 +68,43 @@
>>   * without generating warnings.
>>   */
>>  
>> +/*
>> + * g_memdup2_qemu:
>> + * @mem: (nullable): the memory to copy.
>> + * @byte_size: the number of bytes to copy.
>> + *
>> + * Allocates @byte_size bytes of memory, and copies @byte_size bytes into it
>> + * from @mem. If @mem is %NULL it returns %NULL.
>> + *
>> + * This replaces g_memdup(), which was prone to integer overflows when
>> + * converting the argument from a #gsize to a #guint.
>> + *
>> + * This static inline version is a backport of the new public API from
>> + * GLib 2.68, kept internal to GLib for backport to older stable releases.
>> + * See https://gitlab.gnome.org/GNOME/glib/-/issues/2319.
>> + *
>> + * Returns: (nullable): a pointer to the newly-allocated copy of the memory,
>> + *          or %NULL if @mem is %NULL.
>> + */
>> +static inline gpointer g_memdup2_qemu(gconstpointer mem, gsize byte_size)
>> +{
>> +#if GLIB_CHECK_VERSION(2, 68, 0)
>> +    return g_memdup2(mem, byte_size);
>> +#else
>> +    gpointer new_mem;
>> +
>> +    if (mem && byte_size != 0) {
>> +        new_mem = g_malloc(byte_size);
>> +        memcpy(new_mem, mem, byte_size);
>> +    } else {
>> +        new_mem = NULL;
>> +    }
>> +
>> +    return new_mem;
>> +#endif
>> +}
>> +#define g_memdup2(m, s) g_memdup2_qemu(m, s)
>> +
> 
> As per our style wouldn't it make sense to just call it qemu_memdup(m,
> s)?

I followed the documentation in include/glib-compat.h:

/*
 * Note that because of the GLIB_VERSION_MAX_ALLOWED constant above,
allowing
 * use of functions from newer GLib via this compat header needs a little
 * trickery to prevent warnings being emitted.
 *
 * Consider a function from newer glib-X.Y that we want to use
 *
 *    int g_foo(const char *wibble)
 *
 * We must define a static inline function with the same signature that does
 * what we need, but with a "_qemu" suffix e.g.
 *
 * static inline void g_foo_qemu(const char *wibble)
 * {
 *     #if GLIB_CHECK_VERSION(X, Y, 0)
 *        g_foo(wibble)
 *     #else
 *        g_something_equivalent_in_older_glib(wibble);
 *     #endif
 * }
 *
 * The #pragma at the top of this file turns off -Wdeprecated-declarations,
 * ensuring this wrapper function impl doesn't trigger the compiler warning
 * about using too new glib APIs. Finally we can do
 *
 *   #define g_foo(a) g_foo_qemu(a)
 *
 * So now the code elsewhere in QEMU, which *does* have the
 * -Wdeprecated-declarations warning active, can call g_foo(...) as normal,
 * without generating warnings.
 */

which is how g_unix_get_passwd_entry_qemu() is implemented.

Should we reword the documentation first?