Re: [PATCH 2/2] hw/core/loader: workaround read() size limit.

[Jamie Iles <jamie@nuviainc.com>]

Hi Philippe,

On Thu, Nov 11, 2021 at 03:55:48PM +0100, Philippe Mathieu-Daudé wrote:
> Hi Jamie,
> 
> On 11/11/21 15:11, Jamie Iles wrote:
> > On Linux, read() will only ever read a maximum of 0x7ffff000 bytes
> > regardless of what is asked.  If the file is larger than 0x7ffff000
> > bytes the read will need to be broken up into multiple chunks.
> > 
> > Cc: Luc Michel <lmichel@kalray.eu>
> > Signed-off-by: Jamie Iles <jamie@nuviainc.com>
> > ---
> >  hw/core/loader.c | 40 ++++++++++++++++++++++++++++++++++------
> >  1 file changed, 34 insertions(+), 6 deletions(-)
> > 
> > diff --git a/hw/core/loader.c b/hw/core/loader.c
> > index 348bbf535bd9..16ca9b99cf0f 100644
> > --- a/hw/core/loader.c
> > +++ b/hw/core/loader.c
> > @@ -80,6 +80,34 @@ int64_t get_image_size(const char *filename)
> >      return size;
> >  }
> >  
> > +static ssize_t read_large(int fd, void *dst, size_t len)
> > +{
> > +    /*
> > +     * man 2 read says:
> > +     *
> > +     * On Linux, read() (and similar system calls) will transfer at most
> > +     * 0x7ffff000 (2,147,479,552) bytes, returning the number of bytes
> 
> Could you mention MAX_RW_COUNT from linux/fs.h?
> 
> > +     * actually transferred.  (This is true on both 32-bit and 64-bit
> > +     * systems.)
> 
> Maybe "This is true for both ILP32 and LP64 data models used by Linux"?
> (because that would not be the case for the ILP64 model).
> 
> Otherwise s/systems/Linux variants/?
> 
> > +     *
> > +     * So read in chunks no larger than 0x7ffff000 bytes.
> > +     */
> > +    size_t max_chunk_size = 0x7ffff000;
> 
> We can declare it static const.

Ack, can fix all of those up.

> > +    size_t offset = 0;
> > +
> > +    while (offset < len) {
> > +        size_t chunk_len = MIN(max_chunk_size, len - offset);
> > +        ssize_t br = read(fd, dst + offset, chunk_len);
> > +
> > +        if (br < 0) {
> > +            return br;
> > +        }
> > +        offset += br;
> > +    }
> > +
> > +    return (ssize_t)len;
> > +}
> 
> I see other read()/pread() calls:
> 
> hw/9pfs/9p-local.c:472:            tsize = read(fd, (void *)buf, bufsz);
> hw/vfio/common.c:269:    if (pread(vbasedev->fd, &buf, size,
> region->fd_offset + addr) != size) {
> ...
> 
> Maybe the read_large() belongs to "sysemu/os-xxx.h"?

I think util/osdep.c would be a good fit for this.  To make sure we're 
on the same page though are you proposing converting all pread/read 
calls to a qemu variant or auditing for ones that could potentially take 
a larger size?

Thanks,

Jamie