On 2021-12-08 8:05 a.m., Helge Deller wrote:
On 12/8/21 13:51, Helge Deller wrote:
On 12/7/21 22:47, John David Anglin wrote:
The glibc tst-minsigstksz-5 test fails with a protection id trap.
tst-minsigstksz (pid 19958): Protection id trap (code 7) at 00000000f5b00497
The problem seems to be that the signal return trampoline is placed
on the alternate stack but the alternate is not executable. It is
allocated by malloc.> ...
Stacks are normally executable on hppa so the trampoline works. But
user code wouldn't normally make an alternate stack executable.
True, I think most people just forget that such architectures exist.
Anyway, the glibc testcase is interesting.
The pretty similar sigaltstack testcase from LTP does work:
https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/sigaltstack/sigaltstack01.c
Both use malloc() to allocate the memory, the only difference is the size allocated.
If you change the glibc testcase to:
-- size_t stack_buffer_size = 64 * 1024 * 1024;
++ size_t stack_buffer_size = SIGSTKSZ;
it allocates only 8kB instead of 64MB.
It seems glibc uses brk() in both cases, but when allocating 64MB it additionally adds a mmap() with READ/WRITE permissions only:
brk(0x606000) = 0x606000
mmap2(NULL, 67112960, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xf303f000
This will then break - as you mentioned - the signal handling on parisc.
I see those options to fix it:/usr/include/hppa-linux-gnu/asm/signal.h:#define MINSIGSTKSZ 2048
I wonder about defining MINSIGSTKSZ to 2048 as it is smaller than a page.
mprotect requires a page aligned address. Alternate stack isn't going to be page aligned if it is allocated by
malloc. Malloc alignment isn't sufficient for nominal 64-byte stack alignment specified in runtime.
/usr/include/hppa-linux-gnu/asm/signal.h:#define SIGSTKSZ 8192
a) Fix the application to map the memory +x
Doesn't fix problem..
b) Add some code to glibc to map the memory +x when sigaltstack is called.
See mprotect comment.
c) Add some (parisc-only) code to kernel to set the permission.
Again, I think region needs to be page aligned.
Option d):
Enhance the kernel to create a per-process new page and map it +rx into the userspace
at process start time. Kernel sets up the page to contain the signal trampoline code.
Option e):
Finally implement vDSO, which then includes option d)
With options d) and e) we could get completely rid of executable stacks.
I like the later two options.
Dave
--
John David Anglin dave.anglin@xxxxxxxx
