Re: [PATCH v6 8/8] linux-user: Load pie executables at upper memory

[Helge Deller <deller@gmx.de>]

On 8/2/23 09:49, Akihiko Odaki wrote:
> On 2023/08/02 8:27, Helge Deller wrote:
>> Fix the elf loader to calculate a valid TASK_UNMAPPED_BASE address for all
>> 32-bit architectures, based on the GUEST_ADDR_MAX constant.
>>
>> Additionally modify the elf loader to load dynamic pie executables at
>> around:
>> ~ 0x5500000000  for 64-bit guest binaries on 64-bit host,
>> - 0x00300000    for 32-bit guest binaries on 64-bit host, and
>> - 0x00000000    for 32-bit guest binaries on 32-bit host.
>
> Why do you change guest addresses depending on the host?

The addresses are guest-addresses.
A 32-bit guest PIE can't be loaded at e.g. 0x5500000000,
while a 64-bit guest PIE needs to be loaded at 0x5500000000.

>> With this patch the Thread Sanitizer (TSan) application will work again,
>> as in commit aab613fb9597 ("linux-user: Update TASK_UNMAPPED_BASE for
>> aarch64").
>>
>> Signed-off-by: Helge Deller <deller@gmx.de>
>> ---
>>   linux-user/elfload.c |  6 ++++--
>>   linux-user/loader.h  | 12 ++++++++++++
>>   linux-user/mmap.c    | 35 ++++++++++++++++++-----------------
>>   3 files changed, 34 insertions(+), 19 deletions(-)
>>
>> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
>> index 47a118e430..8f5a79b537 100644
>> --- a/linux-user/elfload.c
>> +++ b/linux-user/elfload.c
>> @@ -3021,6 +3021,7 @@ static void load_elf_image(const char *image_name, int image_fd,
>>       struct elfhdr *ehdr = (struct elfhdr *)bprm_buf;
>>       struct elf_phdr *phdr;
>>       abi_ulong load_addr, load_bias, loaddr, hiaddr, error;
>> +    unsigned long load_offset = 0;
>>       int i, retval, prot_exec;
>>       Error *err = NULL;
>>       bool is_main_executable;
>> @@ -3121,6 +3122,7 @@ static void load_elf_image(const char *image_name, int image_fd,
>>                * select guest_base.  In this case we pass a size.
>>                */
>>               probe_guest_base(image_name, 0, hiaddr - loaddr);
>> +            load_offset = TASK_UNMAPPED_BASE_PIE;
>>           }
>>       }
>>
>> @@ -3138,7 +3140,7 @@ static void load_elf_image(const char *image_name, int image_fd,
>>        * In both cases, we will overwrite pages in this range with mappings
>>        * from the executable.
>>        */
>> -    load_addr = target_mmap(loaddr, (size_t)hiaddr - loaddr + 1, PROT_NONE,
>> +    load_addr = target_mmap(loaddr + load_offset, (size_t)hiaddr - loaddr + 1, PROT_NONE,
>>                               MAP_PRIVATE | MAP_ANON | MAP_NORESERVE |
>>                               (is_main_executable ? MAP_FIXED : 0),
>>                               -1, 0);
>> @@ -3176,7 +3178,7 @@ static void load_elf_image(const char *image_name, int image_fd,
>>       info->start_data = -1;
>>       info->end_data = 0;
>>       /* possible start for brk is behind all sections of this ELF file. */
>> -    info->brk = TARGET_PAGE_ALIGN(hiaddr);
>> +    info->brk = TARGET_PAGE_ALIGN(load_offset + hiaddr);
>>       info->elf_flags = ehdr->e_flags;
>>
>>       prot_exec = PROT_EXEC;
>> diff --git a/linux-user/loader.h b/linux-user/loader.h
>> index 59cbeacf24..3bbfc108eb 100644
>> --- a/linux-user/loader.h
>> +++ b/linux-user/loader.h
>> @@ -18,6 +18,18 @@
>>   #ifndef LINUX_USER_LOADER_H
>>   #define LINUX_USER_LOADER_H
>>
>> +/* where to map binaries? */
>> +#if HOST_LONG_BITS == 64 && TARGET_ABI_BITS == 64
>> +# define TASK_UNMAPPED_BASE_PIE 0x5500000000
>> +# define TASK_UNMAPPED_BASE    0x7000000000
>> +#elif HOST_LONG_BITS == 64 && TARGET_ABI_BITS == 32
>> +# define TASK_UNMAPPED_BASE_PIE    0x00300000
>> +# define TASK_UNMAPPED_BASE    (GUEST_ADDR_MAX - 0x20000000 + 1)
>> +#else /* HOST_LONG_BITS == 32 && TARGET_ABI_BITS == 32 */
>> +# define TASK_UNMAPPED_BASE_PIE    0x00000000
>> +# define TASK_UNMAPPED_BASE    0x40000000
>> +#endif
>> +
>>   /*
>>    * Read a good amount of data initially, to hopefully get all the
>>    * program headers loaded.
>> diff --git a/linux-user/mmap.c b/linux-user/mmap.c
>> index c624feead0..3441198e21 100644
>> --- a/linux-user/mmap.c
>> +++ b/linux-user/mmap.c
>> @@ -23,6 +23,7 @@
>>   #include "user-internals.h"
>>   #include "user-mmap.h"
>>   #include "target_mman.h"
>> +#include "loader.h"
>>
>>   static pthread_mutex_t mmap_mutex = PTHREAD_MUTEX_INITIALIZER;
>>   static __thread int mmap_lock_count;
>> @@ -295,23 +296,6 @@ static bool mmap_frag(abi_ulong real_start, abi_ulong start, abi_ulong last,
>>       return true;
>>   }
>>
>> -#if HOST_LONG_BITS == 64 && TARGET_ABI_BITS == 64
>> -#ifdef TARGET_AARCH64
>> -# define TASK_UNMAPPED_BASE  0x5500000000
>> -#else
>> -# define TASK_UNMAPPED_BASE  0x4000000000
>> -#endif
>> -#elif HOST_LONG_BITS == 64 && TARGET_ABI_BITS == 32
>> -#ifdef TARGET_HPPA
>> -# define TASK_UNMAPPED_BASE  0xfa000000
>> -#else
>> -# define TASK_UNMAPPED_BASE  0xe0000000
>> -#endif
>> -#else /* HOST_LONG_BITS == 32 && TARGET_ABI_BITS == 32 */
>> -# define TASK_UNMAPPED_BASE  0x40000000
>> -#endif
>> -abi_ulong mmap_next_start = TASK_UNMAPPED_BASE;
>> -
>>   unsigned long last_brk;
>>
>>   /*
>> @@ -344,6 +328,23 @@ abi_ulong mmap_find_vma(abi_ulong start, abi_ulong size, abi_ulong align)
>>       abi_ulong addr;
>>       int wrapped, repeat;
>>
>> +    static abi_ulong mmap_next_start;
>> +
>> +    /* initialize mmap_next_start if necessary */
>> +    if (!mmap_next_start) {
>> +        mmap_next_start = TASK_UNMAPPED_BASE;
>> +
>> +        /* do sanity checks on guest memory layout */
>> +        if (mmap_next_start >= GUEST_ADDR_MAX) {
>> +            mmap_next_start = GUEST_ADDR_MAX - 0x1000000000 + 1;
>
> What if GUEST_ADDR_MAX < 0x1000000000?

this check affects 64-bit executables only where GUEST_ADDR_MAX is bigger
than that number. But I agree it's not directly visible from the code.
32-bit ones are taken care of where TASK_UNMAPPED_BASE is defined.

> I think you can just return a hard error when mmap_next_start >= GUEST_ADDR_MAX.

Can't happen, but I will rewrite it.

>> +        }
>> +
>> +        if (TASK_UNMAPPED_BASE_PIE >= mmap_next_start) {
>> +            fprintf(stderr, "Memory too small for PIE executables.\n");
>
> Perhaps it's better to use error_report() for new code.

Ok.

Helge