Re: [Qemu-devel] [PATCH 1/2] elf-ops.h: Map into memory the ELF to load

[Stefano Garzarella]

On Tue, Jul 23, 2019 at 10:50:24AM +0100, Peter Maydell wrote:
> On Tue, 23 Jul 2019 at 10:42, Stefano Garzarella <address@hidden> wrote:
> > Reading the 'g_mapped_file_new_from_fd()' docs [1]:
> > "If writable is TRUE, the mapped buffer may be modified, otherwise it is an
> > error to modify the mapped buffer. Modifications to the buffer are not 
> > visible
> > to other processes mapping the same file, and are not written back to the 
> > file."
> >
> > I don't know what "error" means, but reading the second part I thought
> > the changes in that case were only visible at the current process.
> 
> Ah, I misread the docs here (and thought the following paragraph
> which talks about changes to the underlying file becoming visible
> to the mapping process was talking about changes in the mapping
> process becoming visible to the file).

I misread too...

> 
> So I think the answer is that we do want to pass writable=true.

Yes, I'll do in the v2!

> 
> Looking at the implementation, we always use mmap()'s MAP_PRIVATE,
> so we get a copy-on-write mapping that doesn't change the underlying
> file. The effect of the 'writable' flag is that we use PROT_READ|PROT_WRITE,
> so if we don't pass writable=true we're liable to get a segfault.

Yes, I just tried and I got the segfault.

Thanks,
Stefano