Re: [PATCH 3/3] vmcore: Convert read_from_oldmem() to take an iov_iter

Matthew Wilcox
2021-12-13T13:42:44Z

On Mon, Dec 13, 2021 at 09:02:57AM +0100, Christoph Hellwig wrote:
> >
> > ssize_t elfcorehdr_read(char *buf, size_t count, u64 *ppos)
> > {
> > - return read_from_oldmem(buf, count, ppos, 0,
> > + struct kvec kvec = { .iov_base = buf, .iov_len = count };
> > + struct iov_iter iter;
> > +
> > + iov_iter_kvec(&iter, READ, &kvec, 1, count);
> > +
> > + return read_from_oldmem(&iter, count, ppos,
> > cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT));
> > }
>
> elfcorehdr_read should probably also take an iov_iter while we're at it.

I don't like how that looks. For example:

@@ -1246,11 +1247,14 @@ static int __init parse_crash_elf64_headers(void)
int rc=0;
Elf64_Ehdr ehdr;
u64 addr;
+ struct iov_iter iter;
+ struct kvec kvec = { .iov_base = &ehdr, .iov_len = sizeof(ehdr) };

addr = elfcorehdr_addr;

/* Read Elf header */
- rc = elfcorehdr_read((char *)&ehdr, sizeof(Elf64_Ehdr), &addr);
+ iov_iter_kvec(&iter, READ, &kvec, 1, sizeof(ehdr));
+ rc = elfcorehdr_read(&iter, &addr);
if (rc < 0)
return rc;

@@ -1277,7 +1281,10 @@ static int __init parse_crash_elf64_headers(void)
if (!elfcorebuf)
return -ENOMEM;
addr = elfcorehdr_addr;
- rc = elfcorehdr_read(elfcorebuf, elfcorebuf_sz_orig, &addr);
+ kvec.iov_base = elfcorebuf;
+ kvec.iov_len = elfcorebuf_sz_orig;
+ iov_iter_kvec(&iter, READ, &kvec, 1, elfcorebuf_sz_orig);
+ rc = elfcorehdr_read(&iter, &addr);
if (rc < 0)
goto fail;

I don't think this makes the code clearer, and it's already pretty ugly
code. I'd rather leave constructing the iov_iter to elfcorehdr_read().
(Same remarks apply to elfcorehdr_read_notes())

> I also don't quite understand why we even need the arch overrides for it,
> but that would require some digging into the history of this interface.

I decided I didn't want to know ;-)