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compel/x86: probe kernel task_size at runtime

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Change compel_task_size() to probe the actual kernel address space
limit at runtime via mmap() with MAP_FIXED_NOREPLACE flag.

On systems where the CPU supports 5-level paging but the kernel
doesn't have it enabled, we need to detect the actual task size the
kernel is using.

On x86-64, task_size is either (1UL << 47) for 4-level paging or
(1UL << 56) for 5-level paging. The function probes by attempting
to mmap at the 4-level boundary and if it succeeds or the address
is already occupied, the kernel supports 5-level paging.

Fixes: #2877

Signed-off-by: Tushar Mohapatra <tusharmohapatra.gig@gmail.com>
This commit is contained in:
Tushar Mohapatra 2026-02-16 10:50:16 +05:30 • committed by Andrei Vagin
parent c676864d41
commit 100b3087fc
1 changed files with 21 additions and 2 deletions
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23
compel/arch/x86/src/lib/infect.c
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@ -17,6 +17,7 @@
#include <compel/plugins/std/syscall-codes.h>
#include <compel/plugins/std/syscall.h>
#include "common/err.h"
#include "common/page.h"
#include "asm/infect-types.h"
#include "ptrace.h"
#include "infect.h"
@ -738,7 +739,9 @@ int ptrace_set_regs(pid_t pid, user_regs_struct_t *regs)
return ptrace(PTRACE_SETREGSET, pid, NT_PRSTATUS, &iov);
}
#define TASK_SIZE ((1UL << 47) - PAGE_SIZE)
#define TASK_SIZE_47 ((1UL << 47) - PAGE_SIZE)
#define TASK_SIZE_56 ((1UL << 56) - PAGE_SIZE)
/*
* Task size may be limited to 3G but we need a
* higher limit, because it's backward compatible.
@ -747,7 +750,23 @@ int ptrace_set_regs(pid_t pid, user_regs_struct_t *regs)
unsigned long compel_task_size(void)
{
return TASK_SIZE;
void *addr;
/*
* On x86-64, task_size is either (1UL << 47) for 4-level paging
* or (1UL << 56) for 5-level paging.
*/
addr = mmap((void *)(TASK_SIZE_47), page_size(), PROT_NONE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED_NOREPLACE,
-1, 0);
if (addr != MAP_FAILED) {
munmap(addr, page_size());
return TASK_SIZE_56;
}
if (errno == EEXIST)
return TASK_SIZE_56;
return TASK_SIZE_47;
}
bool __compel_shstk_enabled(user_fpregs_struct_t *ext_regs)