[PATCH 2/6] x86/setup: Rework cmdline_cook() to be compatible with -Wwrite-strings

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Constify both cmdline variables in create_dom0() and __start_xen().
Initialise Xen's variable to the empty string to simplify the parsing logic.

Update cmdline_cook() to take and return const pointers, changing it to have
an early exit for a NULL input (which can happen if the mbi-> pointers happen
to be 0).

Note this only compiles because strstr() launders the const off the pointer
when assigning to the mutable kextra, but that logic only mutates the
mbi->cmdline buffer.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
CC: Roberto Bagnara <roberto.bagnara@xxxxxxxxxxx>
CC: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>

I don't particularly like this, but it's the best I can come up with.
---
 xen/arch/x86/setup.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index c41dfdb2bdf8..a06a241943f6 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -837,9 +837,10 @@ static bool __init loader_is_grub2(const char *loader_name)
     return (p != NULL) && (p[5] != '0');
 }
 
-static char * __init cmdline_cook(char *p, const char *loader_name)
+static const char *__init cmdline_cook(const char *p, const char *loader_name)
 {
-    p = p ? : "";
+    if ( !p )
+        return "";
 
     /* Strip leading whitespace. */
     while ( *p == ' ' )
@@ -885,7 +886,7 @@ static struct domain *__init create_dom0(const module_t 
*image,
         },
     };
     struct domain *d;
-    char *cmdline;
+    const char *cmdline;
     domid_t domid;
 
     if ( opt_dom0_pvh )
@@ -971,8 +972,8 @@ static struct domain *__init create_dom0(const module_t 
*image,
 /* SAF-1-safe */
 void __init noreturn __start_xen(unsigned long mbi_p)
 {
-    const char *memmap_type = NULL, *loader;
-    char *cmdline, *kextra;
+    const char *memmap_type = NULL, *loader, *cmdline = "";
+    char *kextra;
     void *bsp_stack;
     struct cpu_info *info = get_cpu_info(), *bsp_info;
     unsigned int initrdidx, num_parked = 0;
@@ -1027,9 +1028,9 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                                            : "unknown";
 
     /* Parse the command-line options. */
-    cmdline = cmdline_cook((mbi->flags & MBI_CMDLINE) ?
-                           __va(mbi->cmdline) : NULL,
-                           loader);
+    if ( mbi->flags & MBI_CMDLINE )
+        cmdline = cmdline_cook(__va(mbi->cmdline), loader);
+
     if ( (kextra = strstr(cmdline, " -- ")) != NULL )
     {
         /*
-- 
2.30.2