pstore fix:

- Fix corrupted compression due to unlucky size choice with ECC
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 Comment: Kees Cook <kees@outflux.net>
 
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Merge tag 'pstore-v4.20-rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull pstore fix from Kees Cook:
 "Fix corrupted compression due to unlucky size choice with ECC"

* tag 'pstore-v4.20-rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  pstore/ram: Correctly calculate usable PRZ bytes

fs/pstore/ram.c

@@ -816,17 +816,14 @@ static int ramoops_probe(struct platform_device *pdev)
 
 	cxt->pstore.data = cxt;
 	/*
-	 * Console can handle any buffer size, so prefer LOG_LINE_MAX. If we
-	 * have to handle dumps, we must have at least record_size buffer. And
-	 * for ftrace, bufsize is irrelevant (if bufsize is 0, buf will be
-	 * ZERO_SIZE_PTR).
+	 * Since bufsize is only used for dmesg crash dumps, it
+	 * must match the size of the dprz record (after PRZ header
+	 * and ECC bytes have been accounted for).
 	 */
-	if (cxt->console_size)
-		cxt->pstore.bufsize = 1024; /* LOG_LINE_MAX */
-	cxt->pstore.bufsize = max(cxt->record_size, cxt->pstore.bufsize);
-	cxt->pstore.buf = kmalloc(cxt->pstore.bufsize, GFP_KERNEL);
+	cxt->pstore.bufsize = cxt->dprzs[0]->buffer_size;
+	cxt->pstore.buf = kzalloc(cxt->pstore.bufsize, GFP_KERNEL);
 	if (!cxt->pstore.buf) {
-		pr_err("cannot allocate pstore buffer\n");
+		pr_err("cannot allocate pstore crash dump buffer\n");
 		err = -ENOMEM;
 		goto fail_clear;
 	}

include/linux/pstore.h

@@ -90,7 +90,10 @@ struct pstore_record {
  *
  * @buf_lock:	spinlock to serialize access to @buf
  * @buf:	preallocated crash dump buffer
- * @bufsize:	size of @buf available for crash dump writes
+ * @bufsize:	size of @buf available for crash dump bytes (must match
+ *		smallest number of bytes available for writing to a
+ *		backend entry, since compressed bytes don't take kindly
+ *		to being truncated)
  *
  * @read_mutex:	serializes @open, @read, @close, and @erase callbacks
  * @flags:	bitfield of frontends the backend can accept writes for