[brown paperbag] fix coredump breakage

Let me count the ways in which I'd screwed up:

* when emitting a page, handling of gaps in coredump should happen
before fetching the current file position.
* fix for a problem that occurs on rather uncommon setups (and hadn't
been observed in the wild) had been sent very late in the cycle.
* ... with badly insufficient testing, introducing an easily
reproducible breakage.  Without giving it time to soak in -next.

Fucked-up-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: "J. R. Okajima" <hooanon05g@gmail.com>
Tested-by: "J. R. Okajima" <hooanon05g@gmail.com>
Fixes: 06bbaa6dc53c "[coredump] don't use __kernel_write() on kmap_local_page()"
Cc: stable@kernel.org	# v6.0-only
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/coredump.c b/fs/coredump.c
index 1ab4f5b..3538f3a 100644 (file)
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -841,7 +841,7 @@ static int dump_emit_page(struct coredump_params *cprm, struct page *page)
        };
        struct iov_iter iter;
        struct file *file = cprm->file;
-       loff_t pos = file->f_pos;
+       loff_t pos;
        ssize_t n;
 
        if (cprm->to_skip) {
@@ -853,6 +853,7 @@ static int dump_emit_page(struct coredump_params *cprm, struct page *page)
                return 0;
        if (dump_interrupted())
                return 0;
+       pos = file->f_pos;
        iov_iter_bvec(&iter, WRITE, &bvec, 1, PAGE_SIZE);
        n = __kernel_write_iter(cprm->file, &iter, &pos);
        if (n != PAGE_SIZE)