This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH v3] libio: do not unbuffer legacy standard files in compatibility mode [BZ #24228]
- From: Florian Weimer <fweimer at redhat dot com>
- To: "Dmitry V. Levin" <ldv at altlinux dot org>
- Cc: libc-alpha at sourceware dot org
- Date: Wed, 19 Jun 2019 18:15:06 +0200
- Subject: Re: [PATCH v3] libio: do not unbuffer legacy standard files in compatibility mode [BZ #24228]
- References: <20190218124438.GB20127@altlinux.org> <87mumtcl0w.fsf@oldenburg2.str.redhat.com> <20190218191021.GA25527@altlinux.org> <20190219005741.GA29070@altlinux.org> <20190219012913.GB29070@altlinux.org> <87lfxx4t6x.fsf@oldenburg2.str.redhat.com> <20190619160340.GA23394@altlinux.org>
* Dmitry V. Levin:
> On Wed, Jun 19, 2019 at 03:10:14PM +0200, Florian Weimer wrote:
>> * Dmitry V. Levin:
>>
>> > diff --git a/libio/genops.c b/libio/genops.c
>> > index 2a0d9b81df..aa92d61b6b 100644
>> > --- a/libio/genops.c
>> > +++ b/libio/genops.c
>> > @@ -789,6 +789,10 @@ _IO_unbuffer_all (void)
>> >
>> > for (fp = (FILE *) _IO_list_all; fp; fp = fp->_chain)
>> > {
>> > +#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
>> > + if (__glibc_unlikely (&_IO_stdin_used == NULL) && _IO_legacy_file (fp))
>> > + continue;
>> > +#endif
>> > if (! (fp->_flags & _IO_UNBUFFERED)
>> > /* Iff stream is un-orientated, it wasn't used. */
>> > && fp->_mode != 0)
>>
>> I believe a better fix would be this, in case an old-style file showed
>> up for a different reason:
>>
>> #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
>> bool potentially_wide_stream = _IO_vtable_offset (fp) != 0;
>> #else
>> bool potentially_wide_stream = true;
>> #endif
>> if (potentially_wide_stream && fp->_mode > 0)
>> _IO_wsetb (fp, NULL, NULL, 0);
>>
>> This is _IO_new_fclose handles this situation.
>
> Yes, this approach seems to work, too:
>
> diff --git a/libio/genops.c b/libio/genops.c
> index 2a0d9b81df..575f0e6584 100644
> --- a/libio/genops.c
> +++ b/libio/genops.c
> @@ -789,6 +789,10 @@ _IO_unbuffer_all (void)
>
> for (fp = (FILE *) _IO_list_all; fp; fp = fp->_chain)
> {
> +#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
> + if (__glibc_unlikely (_IO_vtable_offset (fp) != 0))
> + continue;
> +#endif
> if (! (fp->_flags & _IO_UNBUFFERED)
> /* Iff stream is un-orientated, it wasn't used. */
> && fp->_mode != 0)
Hmm, right there's an early access to fp->_mode that I had missed.
Should we still flush buffers in old binaries?
I think we could do this instead:
int mode;
#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
if (__glibc_unlikely (_IO_vtable_offset (fp) != 0))
mode = 1; /* Old streams are never wide. */
else
mode = fp->_mode;
#else
mode = fp->_mode;
#endif
And then use mode instead of fp->_mode below. Does this make sense?
>> I fear the test is unreliable because it depends on what fp->_mode
>> evaluates to (which is not actually present in the struct with old
>> files). But the test is definitely better than nothing.
>
> Sorry, I couldn't think of a more reliable test than that.
I came up with this (also with the linker script). But curiously
enough, the padding is not actually needed. The test is only valid with
GLIBC_2.0 targets, it crashes for newer targets which do not define
_IO_stdout_.
#include <stdio.h>
#include <stdlib.h>
int pad1[] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 };
int _IO_stdout_[20] __attribute__ ((nocommon));
int pad2[] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 };
int
main (void)
{
/* Simulate old-style printf. */
fprintf ((FILE *) &_IO_stdout_, "info: testing old printing\n");
return 0;
}
This could be improved by using internal headers when the test is built
within the glibc tree, so that hard-coding the size of _IO_stdout_ is
not required.
I guess we could add both tests, just in case.
Thanks,
Florian