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Subject[PATCH] arm64: Expose TASK_SIZE to userspace via auxv
Date
Some userspace applications need to know the maximum virtual address they can
use (TASK_SIZE). There are several possible values for TASK_SIZE with the arm64
kernel, and such applications are either making bad hard-coded assumptions, or
are guessing and checking using system calls like munmap(), which may have
other reasons for returning an error than TASK_SIZE being exceeded. To make
correct functioning easy for userspace applications that need to know the
maximum virtual address they can use, communicate TASK_SIZE via the ELF
auxiliary vector, just like PAGE_SIZE is currently communicated.

Signed-off-by: Christopher Covington <cov@codeaurora.org>
---
Tested with the following commands:
LD_SHOW_AUXV=1 sleep 1 # GNU dynamic ld-linux*.so
hexdump -v -e '4/4 "%08x " "\n"' /proc/self/auxv | \
sed -r 's/0*([^ ]+) ([^ ]+) ([^ ]+) ([^ ]+)/\1 0x\4\3/
s/^0 / NULL: /
s/^3 / PHDR: /
s/^4 / PHENT: /
s/^5 / PHNUM: /
s/^6 / PAGESZ: /
s/^7 / BASE: /
s/^8 / FLAGS: /
s/^9 / ENTRY: /
s/^b / UID: /
s/^c / EUID: /
s/^d / GID: /
s/^e / EGID: /
s/^f /PLATFORM: /
s/^10 / HWCAP: /
s/^11 / CLKTCK: /
s/^17 / SECURE: /
s/^19 / RANDOM: /
s/^1f / EXECFN: /
s/^21 / VDSO: /
s/^22 / TASKSZ: /' # compatible with static busybox
---
arch/arm64/include/asm/elf.h | 1 +
arch/arm64/include/uapi/asm/auxvec.h | 3 ++-
2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index a55384f..3811795 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -145,6 +145,7 @@ typedef struct user_fpsimd_state elf_fpregset_t;
do { \
NEW_AUX_ENT(AT_SYSINFO_EHDR, \
(elf_addr_t)current->mm->context.vdso); \
+ NEW_AUX_ENT(AT_TASKSZ, TASK_SIZE); \
} while (0)

#define ARCH_HAS_SETUP_ADDITIONAL_PAGES
diff --git a/arch/arm64/include/uapi/asm/auxvec.h b/arch/arm64/include/uapi/asm/auxvec.h
index 4cf0c17..595bfda 100644
--- a/arch/arm64/include/uapi/asm/auxvec.h
+++ b/arch/arm64/include/uapi/asm/auxvec.h
@@ -18,7 +18,8 @@

/* vDSO location */
#define AT_SYSINFO_EHDR 33
+#define AT_TASKSZ 34

-#define AT_VECTOR_SIZE_ARCH 1 /* entries in ARCH_DLINFO */
+#define AT_VECTOR_SIZE_ARCH 2 /* entries in ARCH_DLINFO */

#endif
--
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm
Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora
Forum, a Linux Foundation Collaborative Project.
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