On Thu, 29 Feb 2024 at 09:32, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> One option might be to make a failed memcpy_from_iter_mc() set another
> flag in the iter, and then make fault_in_iov_iter_readable() test that
> flag and return 'len' if that flag is set.
>
> Something like that (wild handwaving) should get the right error handling.
>
> The simpler alternative is maybe something like the attached.
> COMPLETELY UNTESTED. Maybe I've confused myself with all the different
> indiraction mazes in the iov_iter code.
Actually, I think the right model is to get rid of that horrendous
.copy_mc field entirely.
We only have one single place that uses it - that nasty core dumping
code. And that code is *not* performance critical.
And not only isn't it performance-critical, it already does all the
core dumping one page at a time because it doesn't want to write pages
that were never mapped into user space.
So what we can do is
(a) make the core dumping code *copy* the page to a good location
with copy_mc_to_kernel() first
(b) remove this horrendous .copy_mc crap entirely from iov_iter
This is slightly complicated by the fact that copy_mc_to_kernel() may
not even exist, and architectures that don't have it don't want the
silly extra copy. So we need to abstract the "copy to temporary page"
code a bit. But that's probably a good thing anyway in that it forces
us to have nice interfaces.
End result: something like the attached.
AGAIN: THIS IS ENTIRELY UNTESTED.
But hey, so was clearly all the .copy_mc code too that this removes, so...
Linus
fs/coredump.c | 41 ++++++++++++++++++++++++++++++++++++++---
include/linux/uio.h | 16 ----------------
lib/iov_iter.c | 23 -----------------------
3 files changed, 38 insertions(+), 42 deletions(-)
diff --git a/fs/coredump.c b/fs/coredump.c
index f258c17c1841..6a9b9f3280d8 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -872,6 +872,9 @@ static int dump_emit_page(struct coredump_params *cprm, struct page *page)
loff_t pos;
ssize_t n;
+ if (!page)
+ return 0;
+
if (cprm->to_skip) {
if (!__dump_skip(cprm, cprm->to_skip))
return 0;
@@ -884,7 +887,6 @@ static int dump_emit_page(struct coredump_params *cprm, struct page *page)
pos = file->f_pos;
bvec_set_page(&bvec, page, PAGE_SIZE, 0);
iov_iter_bvec(&iter, ITER_SOURCE, &bvec, 1, PAGE_SIZE);
- iov_iter_set_copy_mc(&iter);
n = __kernel_write_iter(cprm->file, &iter, &pos);
if (n != PAGE_SIZE)
return 0;
@@ -895,10 +897,40 @@ static int dump_emit_page(struct coredump_params *cprm, struct page *page)
return 1;
}
+/*
+ * If we might get machine checks from kernel accesses during the
+ * core dump, let's get those errors early rather than during the
+ * IO. This is not performance-critical enough to warrant having
+ * all the machine check logic in the iovec paths.
+ */
+#ifdef copy_mc_to_kernel
+
+#define dump_page_alloc() alloc_page(GFP_KERNEL)
+#define dump_page_free(x) __free_page(x)
+static struct page *dump_page_copy(struct page *src, struct page *dst)
+{
+ void *buf = kmap_local_page(src);
+ size_t left = copy_mc_to_kernel(page_address(dst), buf, PAGE_SIZE);
+ kunmap_local(buf);
+ return left ? NULL : dst;
+}
+
+#else
+
+#define dump_page_alloc() ((struct page *)8) // Not NULL
+#define dump_page_free(x) do { } while (0)
+#define dump_page_copy(src,dst) ((dst),(src))
+
+#endif
+
int dump_user_range(struct coredump_params *cprm, unsigned long start,
unsigned long len)
{
unsigned long addr;
+ struct page *dump_page = dump_page_alloc();
+
+ if (!dump_page)
+ return 0;
for (addr = start; addr < start + len; addr += PAGE_SIZE) {
struct page *page;
@@ -912,14 +944,17 @@ int dump_user_range(struct coredump_params *cprm, unsigned long start,
*/
page = get_dump_page(addr);
if (page) {
- int stop = !dump_emit_page(cprm, page);
+ int stop = !dump_emit_page(cprm, dump_page_copy(page, dump_page));
put_page(page);
- if (stop)
+ if (stop) {
+ dump_page_free(dump_page);
return 0;
+ }
} else {
dump_skip(cprm, PAGE_SIZE);
}
}
+ dump_page_free(dump_page);
return 1;
}
#endif
diff --git a/include/linux/uio.h b/include/linux/uio.h
index bea9c89922d9..00cebe2b70de 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -40,7 +40,6 @@ struct iov_iter_state {
struct iov_iter {
u8 iter_type;
- bool copy_mc;
bool nofault;
bool data_source;
size_t iov_offset;
@@ -248,22 +247,8 @@ size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i);
#ifdef CONFIG_ARCH_HAS_COPY_MC
size_t _copy_mc_to_iter(const void *addr, size_t bytes, struct iov_iter *i);
-static inline void iov_iter_set_copy_mc(struct iov_iter *i)
-{
- i->copy_mc = true;
-}
-
-static inline bool iov_iter_is_copy_mc(const struct iov_iter *i)
-{
- return i->copy_mc;
-}
#else
#define _copy_mc_to_iter _copy_to_iter
-static inline void iov_iter_set_copy_mc(struct iov_iter *i) { }
-static inline bool iov_iter_is_copy_mc(const struct iov_iter *i)
-{
- return false;
-}
#endif
size_t iov_iter_zero(size_t bytes, struct iov_iter *);
@@ -355,7 +340,6 @@ static inline void iov_iter_ubuf(struct iov_iter *i, unsigned int direction,
WARN_ON(direction & ~(READ | WRITE));
*i = (struct iov_iter) {
.iter_type = ITER_UBUF,
- .copy_mc = false,
.data_source = direction,
.ubuf = buf,
.count = count,
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index e0aa6b440ca5..cf2eb2b2f983 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -166,7 +166,6 @@ void iov_iter_init(struct iov_iter *i, unsigned int direction,
WARN_ON(direction & ~(READ | WRITE));
*i = (struct iov_iter) {
.iter_type = ITER_IOVEC,
- .copy_mc = false,
.nofault = false,
.data_source = direction,
.__iov = iov,
@@ -244,27 +243,9 @@ size_t _copy_mc_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
EXPORT_SYMBOL_GPL(_copy_mc_to_iter);
#endif /* CONFIG_ARCH_HAS_COPY_MC */
-static __always_inline
-size_t memcpy_from_iter_mc(void *iter_from, size_t progress,
- size_t len, void *to, void *priv2)
-{
- return copy_mc_to_kernel(to + progress, iter_from, len);
-}
-
-static size_t __copy_from_iter_mc(void *addr, size_t bytes, struct iov_iter *i)
-{
- if (unlikely(i->count < bytes))
- bytes = i->count;
- if (unlikely(!bytes))
- return 0;
- return iterate_bvec(i, bytes, addr, NULL, memcpy_from_iter_mc);
-}
-
static __always_inline
size_t __copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)
{
- if (unlikely(iov_iter_is_copy_mc(i)))
- return __copy_from_iter_mc(addr, bytes, i);
return iterate_and_advance(i, bytes, addr,
copy_from_user_iter, memcpy_from_iter);
}
@@ -633,7 +614,6 @@ void iov_iter_kvec(struct iov_iter *i, unsigned int direction,
WARN_ON(direction & ~(READ | WRITE));
*i = (struct iov_iter){
.iter_type = ITER_KVEC,
- .copy_mc = false,
.data_source = direction,
.kvec = kvec,
.nr_segs = nr_segs,
@@ -650,7 +630,6 @@ void iov_iter_bvec(struct iov_iter *i, unsigned int direction,
WARN_ON(direction & ~(READ | WRITE));
*i = (struct iov_iter){
.iter_type = ITER_BVEC,
- .copy_mc = false,
.data_source = direction,
.bvec = bvec,
.nr_segs = nr_segs,
@@ -679,7 +658,6 @@ void iov_iter_xarray(struct iov_iter *i, unsigned int direction,
BUG_ON(direction & ~1);
*i = (struct iov_iter) {
.iter_type = ITER_XARRAY,
- .copy_mc = false,
.data_source = direction,
.xarray = xarray,
.xarray_start = start,
@@ -703,7 +681,6 @@ void iov_iter_discard(struct iov_iter *i, unsigned int direction, size_t count)
BUG_ON(direction != READ);
*i = (struct iov_iter){
.iter_type = ITER_DISCARD,
- .copy_mc = false,
.data_source = false,
.count = count,
.iov_offset = 0
