[PATCH v3 2/2] s390: Warn if kernel command line contains non-printable EBCDIC characters

[Ilya Leoshkevich]

Users may accidentally add multi-byte UTF-8 characters to zipl.conf
parmline, for example, by copying snippets containing non-breaking
spaces (\xC2\xA0) from web pages.

The kernel will then interpret the entire command line as EBCDIC,
making it unusable. Distinguish this situation from the legitimate
EBCDIC conversion by looking for non-printable characters and issue
a warning.

The current solution deliberately makes the following tradeoffs:

* Do not make this a hard failure, there may be a very small number
  of users who put characters with diacritics on their EBCDIC-encoded
  command lines.
* Do not use heuristics with arbitrary thresholds, these may fail
  intermittently on, e.g., punctuation-heavy command lines, and having
  to tune thresholds in subsequent patches is not desirable.
* Use the invariant subset of EBCDIC to determine whether characters
  are printable, the vast majority of command lines will contain
  characters from this subset.

Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
---
 arch/s390/boot/ipl_parm.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/s390/boot/ipl_parm.c b/arch/s390/boot/ipl_parm.c
index 6bc950b92be76..24573307263ca 100644
--- a/arch/s390/boot/ipl_parm.c
+++ b/arch/s390/boot/ipl_parm.c
@@ -173,12 +173,23 @@ static inline int has_ebcdic_char(const char *str)
 	return 0;
 }
 
+static inline bool has_nonprintable_ebcdic_char(const char *str)
+{
+	for (; *str; str++)
+		if (!isprint_ebc_inv(*str))
+			return true;
+	return false;
+}
+
 void setup_boot_command_line(void)
 {
 	parmarea.command_line[COMMAND_LINE_SIZE - 1] = 0;
 	/* convert arch command line to ascii if necessary */
-	if (has_ebcdic_char(parmarea.command_line))
+	if (has_ebcdic_char(parmarea.command_line)) {
+		if (has_nonprintable_ebcdic_char(parmarea.command_line))
+			boot_warn("Kernel command line was treated as EBCDIC, but contains non-printable characters\n");
 		EBCASC(parmarea.command_line, COMMAND_LINE_SIZE);
+	}
 	/* copy arch command line */
 	strscpy(early_command_line, strim(parmarea.command_line));
 
-- 
2.55.0