[PATCH] subdom: Fix -Werror=address failure in tmp_emulator

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The opensuse-tumbleweed build jobs currently fail with:

  /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.c: In function 
'rsa_private':
  /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.c:56:7: error: 
the comparison will always evaluate as 'true' for the address of 'p' will never 
be NULL [-Werror=address]
     56 |   if (!key->p || !key->q || !key->u) {
        |       ^
  In file included from 
/builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.c:17:
  /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.h:28:12: note: 
'p' declared here
     28 |   tpm_bn_t p;
        |            ^

This is because all tpm_bn_t's are 1-element arrays (of either a GMP or
OpenSSL BIGNUM flavour).  The author was probably meaning to do value checks,
but that's not what the code does.

Adjust it to compile.  No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
CC: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
CC: Jason Andryuk <jandryuk@xxxxxxxxx>
CC: Daniel Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
CC: Christopher Clark <christopher.w.clark@xxxxxxxxx>

While I've confirmed this to fix the build issue:

  https://gitlab.com/xen-project/people/andyhhp/xen/-/pipelines/955160430

I'm -1 overall to the change, and would prefer to disable vtpm-stubdom
entirely.

It's TPM 1.2 only, using decades-old libs, and some stuff in the upstream
https://github.com/PeterHuewe/tpm-emulator (which is still abandaonded as of
2018) is just as concerning as the basic error here in rsa_private().

vtpm-stubdom isn't credibly component of a Xen system, and we're wasting loads
of CI cycles testing it...
---
 stubdom/Makefile                 |  1 +
 stubdom/vtpm-tpm_bn_t-addr.patch | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+)
 create mode 100644 stubdom/vtpm-tpm_bn_t-addr.patch

diff --git a/stubdom/Makefile b/stubdom/Makefile
index a21e1c3fa3a8..d5fb354e7e37 100644
--- a/stubdom/Makefile
+++ b/stubdom/Makefile
@@ -243,6 +243,7 @@ tpm_emulator-$(XEN_TARGET_ARCH): 
tpm_emulator-$(TPMEMU_VERSION).tar.gz
        patch -d $@ -p1 < vtpm_extern.patch
        patch -d $@ -p1 < vtpm-microsecond-duration.patch
        patch -d $@ -p1 < vtpm-command-duration.patch
+       patch -d $@ -p1 < vtpm-tpm_bn_t-addr.patch
        mkdir $@/build
        cd $@/build; CC=${CC} $(CMAKE) .. -DCMAKE_C_FLAGS:STRING="-std=c99 
-DTPM_NO_EXTERN $(TARGET_CPPFLAGS) $(TARGET_CFLAGS) 
-Wno-declaration-after-statement"
        touch $@
diff --git a/stubdom/vtpm-tpm_bn_t-addr.patch b/stubdom/vtpm-tpm_bn_t-addr.patch
new file mode 100644
index 000000000000..53172ae1c244
--- /dev/null
+++ b/stubdom/vtpm-tpm_bn_t-addr.patch
@@ -0,0 +1,18 @@
+All tpm_bn_t's are a 1-element array of one form or another, meaning the code
+below is tautological and triggers -Werror=address.
+
+diff -ru tpm_emulator-x86_64.orig/crypto/rsa.c tpm_emulator-x86_64/crypto/rsa.c
+--- tpm_emulator-x86_64.orig/crypto/rsa.c      2011-12-20 18:30:06.000000000 
+0000
++++ tpm_emulator-x86_64/crypto/rsa.c   2023-08-03 20:44:17.379166284 +0100
+@@ -53,10 +53,7 @@
+   tpm_bn_init2(c, key->size);
+   tpm_bn_import(p, in_len, 1, in);
+ 
+-  if (!key->p || !key->q || !key->u) {
+-    /* c = p ^ d mod n */
+-    tpm_bn_powm(c, p, key->d, key->n);
+-  } else {
++  {
+     tpm_bn_init2(m1, key->size / 2);
+     tpm_bn_init2(m2, key->size / 2);
+     tpm_bn_init2(h, key->size);

base-commit: 092cae024ab6cd9bd5788eb6ca3ae1a05e796c0a
-- 
2.30.2