[PATCH v1] v4l2: v4l2_compat: Fix redirect from `__open(at)64_2()`

[Barnabás Pőcze]

Hi


2024. június 18., kedd 0:37 keltezéssel, Kieran Bingham <kieran.bingham at ideasonboard.com> írta:

> Quoting Barnabás Pőcze (2024-06-17 22:43:45)
> > `__open64_2()` and `__openat64_2()` should redirect
> > to `open64()` and `openat64()`, respectively, otherwise
> > the `O_LARGEFILE` flag will not be applied.
> >
> > Fixes: 1023107b6405 ("v4l2: v4l2_compat: Intercept open64, openat64, and mmap64")
> > Signed-off-by: Barnabás Pőcze <pobrn at protonmail.com>
> 
> CI is all green here.
> https://gitlab.freedesktop.org/camera/libcamera/-/pipelines/1203630
> 
> But commit 1023107b6405 ("v4l2: v4l2_compat: Intercept open64, openat64, and mmap64")
> states:
> 
>     Also, since we set _FILE_OFFSET_BITS to 32 to force the various open and
>     mmap symbols that we export to not be the 64-bit versions, our dlsym to
>     get the original open and mmap calls will not automatically be converted
>     to their 64-bit versions. Since we intercept both 32-bit and 64-bit
>     versions of open and mmap, we should be using the 64-bit version to
>     service both. Fetch the 64-bit versions of openat and mmap directly.
> 
> is that why these were the non-64 bit variants?
> 
> This feels like it's "too obvious" ... I wonder what we missed ...
> 
> Paul? Any insights? (It was ... a long time ago that you wrote that
> patch)

Looks like it is me who is confused. V4L2CompatManager::instance()->openat()
indeed uses openat64 to service the call. And it would appear that both
`openat64()` and `open64()` unconditionally add `O_LARGEFILE`, which, in hindsight,
is the expected behaviour: https://codebrowser.dev/glibc/glibc/sysdeps/unix/sysv/linux/openat64.c.html#39

In this case, patch does not change a thing, apart from eliminating the
visual inconsistency that caught my eyes in the first place.

However, this raises a different issue: a call to open()/openat() will automatically get `O_LARGEFILE`:

$ cat asd.c
#include <assert.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
	assert(argc == 2);
	return close(open(argv[1], O_RDONLY));
}
$ gcc -m32 asd.c
$ truncate -s 10G testfile

$ strace -e openat ./a.out testfile
[...]
openat(AT_FDCWD, "testfile", O_RDONLY)  = -1 EOVERFLOW (Value too large for defined data type)
+++ exited with 255 +++

$ strace -e openat -E LD_PRELOAD=.../src/v4l2/v4l2-compat.so ./a.out testfile
[...]
openat(AT_FDCWD, "testfile", O_RDONLY|O_LARGEFILE) = 3
+++ exited with 0 +++

So a file that shouldn't be opened can now be opened.


Regards,
Barnabás Pőcze

> 
> 
> 
> > ---
> >  src/v4l2/v4l2_compat.cpp | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/src/v4l2/v4l2_compat.cpp b/src/v4l2/v4l2_compat.cpp
> > index 8e2b7e92..8a44403e 100644
> > --- a/src/v4l2/v4l2_compat.cpp
> > +++ b/src/v4l2/v4l2_compat.cpp
> > @@ -59,7 +59,7 @@ LIBCAMERA_PUBLIC int open64(const char *path, int oflag, ...)
> >
> >  LIBCAMERA_PUBLIC int __open64_2(const char *path, int oflag)
> >  {
> > -       return open(path, oflag);
> > +       return open64(path, oflag);
> >  }
> >  #endif
> >
> > @@ -90,7 +90,7 @@ LIBCAMERA_PUBLIC int openat64(int dirfd, const char *path, int oflag, ...)
> >
> >  LIBCAMERA_PUBLIC int __openat64_2(int dirfd, const char *path, int oflag)
> >  {
> > -       return openat(dirfd, path, oflag);
> > +       return openat64(dirfd, path, oflag);
> >  }
> >  #endif
> >
> > --
> > 2.45.2
> >
> >
>