Re: [PATCH] x86/PVH: account for module command line length

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 19/03/2025 11:18 am, Jan Beulich wrote:
> As per observation in practice, initrd->cmdline_pa is not normally zero.
> Hence so far we always appended at least one byte. That alone may
> already render insufficient the "allocation" made by find_memory().
> Things would be worse when there's actually a (perhaps long) command
> line.
>
> Skip setup when the command line is empty. Amend the "allocation" size
> by padding and actual size of module command line.
>
> Fixes: 0ecb8eb09f9f ("x86/pvh: pass module command line to dom0")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/arch/x86/hvm/dom0_build.c
> +++ b/xen/arch/x86/hvm/dom0_build.c
> @@ -712,7 +712,15 @@ static int __init pvh_load_kernel(
>       * simplify it.
>       */
>      last_addr = find_memory(d, &elf, sizeof(start_info) +
> -                            (initrd ? ROUNDUP(initrd_len, PAGE_SIZE) +
> +                            (initrd ? ROUNDUP(ROUNDUP(initrd_len,
> +                                                      elf_64bit(&elf) ? 8 : 
> 4) +
> +                                              (initrd->cmdline_pa &&
> +                                               strlen(__va(initrd->
> +                                                           cmdline_pa))
> +                                               ? strlen(__va(initrd->
> +                                                             cmdline_pa)) + 1
> +                                               : 0),
> +                                              PAGE_SIZE) +
>                                        sizeof(mod)
>                                      : 0) +
>                              (cmdline ? ROUNDUP(strlen(cmdline) + 1,

This piece of logic was already bad, but this is rather worse.

One patch I made while doing the boot module work is:

diff --git a/xen/arch/x86/hvm/dom0_build.c b/xen/arch/x86/hvm/dom0_build.c
index 6a4453103a9a..7292ddd07276 100644
--- a/xen/arch/x86/hvm/dom0_build.c
+++ b/xen/arch/x86/hvm/dom0_build.c
@@ -654,6 +654,7 @@ static int __init pvh_load_kernel(
     const char *cmdline = image->cmdline_pa ? __va(image->cmdline_pa) :
NULL;
     struct elf_binary elf;
     struct elf_dom_parms parms;
+    size_t metadata_len;
     paddr_t last_addr;
     struct hvm_start_info start_info = { 0 };
     struct hvm_modlist_entry mod = { 0 };
@@ -711,13 +712,16 @@ static int __init pvh_load_kernel(
      * split into smaller allocations, done as a single region in order to
      * simplify it.
      */
-    last_addr = find_memory(d, &elf, sizeof(start_info) +
-                            (initrd ? ROUNDUP(initrd_len, PAGE_SIZE) +
-                                      sizeof(mod)
-                                    : 0) +
-                            (cmdline ? ROUNDUP(strlen(cmdline) + 1,
-                                               elf_64bit(&elf) ? 8 : 4)
-                                     : 0));
+    metadata_len = sizeof(start_info);
+
+    if ( initrd )
+        metadata_len += sizeof(mod) + ROUNDUP(initrd_len, PAGE_SIZE);
+
+    if ( cmdline )
+        metadata_len += ROUNDUP(strlen(cmdline) + 1,
+                                elf_64bit(&elf) ? 8 : 4);
+
+    last_addr = find_memory(d, &elf, metadata_len);
     if ( last_addr == INVALID_PADDR )
     {
         printk("Unable to find a memory region to load initrd and
metadata\n");


which I think I ought to submit as a prerequisite to this, after which
your logic squashed on the RHS now becomes an expansion of the `if (
initrd )`.

Thoughts?

~Andrew