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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 19/03/2025 12:39 pm, Jan Beulich wrote:
> On 19.03.2025 13:12, Andrew Cooper wrote:
>> 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 )`.
> Yes please.

https://lore.kernel.org/xen-devel/20250319171346.19478-1-andrew.cooper3@xxxxxxxxxx/T/#u

> Another thing I was considering to put in a local variable is the
> repeated "elf_64bit(&elf) ? 8 : 4". If that sounds okay, I can make that
> a(nother) pre-patch to mine.

Yeah, I was thinking the same.  There's a related expression in
elf_round_up(), so it might be worth having a macro in
include/xen/libelf.h for elf_ptr_size() or so.

~Andrew