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Add riscv64 support #19

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0000b6b
Add seccomp mode support for riscv64
rota1001 Apr 11, 2026
0000601
Ignore size checking for non-LOAD sections in ELF
rota1001 Apr 14, 2026
0000b08
Add trap mode support for riscv64
rota1001 Apr 14, 2026
00000b6
Update unit test for riscv64
rota1001 Apr 11, 2026
0000392
Note the riscv64 rewrite mode limitation
rota1001 Apr 12, 2026
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1 change: 1 addition & 0 deletions README.md
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Expand Up @@ -267,6 +267,7 @@ with

- x86_64
- aarch64
- riscv64 (Only the trap mode and the seccomp mode are available)

## License

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2 changes: 2 additions & 0 deletions docs/architecture.md
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Expand Up @@ -174,6 +174,8 @@ host-kernel calls. `mmap` and `epoll_*` are deliberately excluded
because they need W^X enforcement, shadow validation, or FD-table
gating that the fast-path cannot perform.

On **riscv64**, the rewrite mode is not available now.

Each site is classified as WRAPPER (simple `syscall; ret` pattern,
eligible for inline virtualized return: getpid=1, gettid=1, getppid=0)
or COMPLEX (result consumed internally by helpers like `raise()` that
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5 changes: 5 additions & 0 deletions mk/toolchain.mk
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Expand Up @@ -29,6 +29,11 @@ CFLAGS += -Wno-unused-parameter
CFLAGS += -Iinclude -Isrc
LDFLAGS += -Wl,-z,noexecstack -Wl,-z,separate-code

# Disable link relaxation of riscv64 architecture to prevent long link time
ifeq ($(ARCH),riscv64)
LDFLAGS += -Wl,--no-relax
endif

# Build mode from Kconfig (fallback to BUILD= for unconfigured builds)
ifeq ($(CONFIG_BUILD_RELEASE),y)
CFLAGS += -O2 -DNDEBUG
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1 change: 1 addition & 0 deletions scripts/alpine-sha256.txt
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@@ -1,2 +1,3 @@
55ea3e5a7c2c35e6268c5dcbb8e45a9cd5b0e372e7b4e798499a526834f7ed90 alpine-minirootfs-3.21.0-x86_64.tar.gz
f31202c4070c4ef7de9e157e1bd01cb4da3a2150035d74ea5372c5e86f1efac1 alpine-minirootfs-3.21.0-aarch64.tar.gz
b2c5ed2be586aebd2da5dd13dbc96bc8cc41b72e517d0726dfbbb0a9810e66d6 alpine-minirootfs-3.21.0-riscv64.tar.gz
2 changes: 2 additions & 0 deletions src/dispatch-misc.c
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Expand Up @@ -239,6 +239,8 @@ struct kbox_dispatch forward_uname(const struct kbox_syscall_request *req,
snprintf(uts.machine, sizeof(uts.machine), "x86_64");
#elif defined(__aarch64__)
snprintf(uts.machine, sizeof(uts.machine), "aarch64");
#elif (defined(__riscv) && __riscv_xlen == 64)
snprintf(uts.machine, sizeof(uts.machine), "riscv64");
#else
snprintf(uts.machine, sizeof(uts.machine), "unknown");
#endif
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2 changes: 1 addition & 1 deletion src/elf.c
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Expand Up @@ -417,7 +417,7 @@ int kbox_build_elf_load_plan(const unsigned char *buf,
p_memsz = read_le64(buf + off + P_MEMSZ_OFF);
p_align = read_le64(buf + off + P_ALIGN_OFF);

if (p_filesz > p_memsz)
if (p_memsz && p_filesz > p_memsz)
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@visitorckw visitorckw Apr 14, 2026

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Looks like a drive-by fix that is not mentioned in the commit message?

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@rota1001 rota1001 Apr 14, 2026

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This is because only those PT_LOAD sections which has non-zero p_memsz will be loaded into the memory. However, the p_filesz > p_memsz will be checked for every section. Thus, those sections with p_memsz (,such as .riscv.attributes,) will make it fail. This checks whether the p_memsz is zero before comparing the p_filesz and the p_memsz.

I have added this to the commit message.

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@visitorckw visitorckw Apr 14, 2026

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I'm not an expert in the ELF parser area, but based on reading the commit message, I have the following questions:

  1. Is this a riscv specific issue, or has it always existed? From the new commit message, it isn't immediately clear to me why this problem is unique to riscv.

  2. It looks very much like this commit is doing two separate things:
    a) Fixing a bug
    b) Adding support for a new feature
    Should these be split into separate commits? If not, could you please explain the reasoning behind keeping them together?

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@rota1001 rota1001 Apr 14, 2026 •
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Is this a riscv specific issue, or has it always existed?

It is not a riscv specific issue. However, so far, it has only been triggered by riscv busybox.

The things that kbox_build_elf_load_plan do are the following two things:

  • Parse flags in non-LOAD sections
  • Find out all the LOAD sections, and output their informations (such as base and size)

All the non-LOAD sections are filtered out with the following check[1]:

if (p_type != PT_LOAD || p_memsz == 0)
    continue;

However, all the sections (,including non-LOAD sections,) will be checked with the following if statement[2]:

if (p_filesz > p_memsz)
    return -1;

Thus, if there is any non-LOAD sections with p_memsz equals to 0 and p_filesz bigger than 0, the kbox_build_elf_load_plan will fail. The reason why this error was not found is this kind of section was only found in riscv so far.

It looks very much like this commit is doing two separate things

OK, I separated them in rebased commits.

[1] https://github.com/sysprog21/kbox/blob/main/src/elf.c#L452C9-L453C22
[2] https://github.com/sysprog21/kbox/blob/main/src/elf.c#L420

Here are some additional descriptions about ELF sections.
A possible question is:

Why there are some non-LOAD sections whose p_memsz are non-zero

This is because they will actually be loaded into the memory. Using the following commands can see the informations about sections:

$ readelf -l busybox
  ...
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000000040 0x0000000000000040
                 0x0000000000000230 0x0000000000000230  R      0x8
 ...
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x00000000000c59a0 0x00000000000c59a0  R E    0x1000

It is found that a section with type PHDR is a non-LOAD section with memsz greater than 0, and its range is totally included by the LOAD section below it. This is the reason why the PHDR section should be loaded into the memory but it was skipped during kbox_build_elf_load_plan.

return -1;

/* Validate that p_offset + p_filesz does not overflow. We do
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4 changes: 2 additions & 2 deletions src/image.c
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Expand Up @@ -490,8 +490,8 @@ static const struct kbox_host_nrs *select_host_nrs(void)
{
#if defined(__x86_64__)
return &HOST_NRS_X86_64;
#elif defined(__aarch64__)
return &HOST_NRS_AARCH64;
#elif defined(__aarch64__) || (defined(__riscv) && (__riscv_xlen == 64))
return &HOST_NRS_GENERIC;
#else
return NULL;
#endif
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3 changes: 3 additions & 0 deletions src/loader-entry.c
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Expand Up @@ -17,6 +17,9 @@ static int machine_to_entry_arch(uint16_t machine,
case 0xb7:
*arch_out = KBOX_LOADER_ENTRY_ARCH_AARCH64;
return 0;
case 0xf3:
*arch_out = KBOX_LOADER_ENTRY_ARCH_RISCV64;
return 0;
default:
return -1;
}
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1 change: 1 addition & 0 deletions src/loader-entry.h
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Expand Up @@ -9,6 +9,7 @@
enum kbox_loader_entry_arch {
KBOX_LOADER_ENTRY_ARCH_X86_64,
KBOX_LOADER_ENTRY_ARCH_AARCH64,
KBOX_LOADER_ENTRY_ARCH_RISCV64
};

struct kbox_loader_entry_state {
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18 changes: 18 additions & 0 deletions src/loader-transfer.c
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Expand Up @@ -89,6 +89,24 @@ kbox_loader_transfer_to_guest(const struct kbox_loader_transfer_state *state)
: "r"(sp), "r"(x0), "r"(x1), "r"(x2), "r"(x3), "r"(x4), "r"(x5),
"r"(x16)
: "memory");
#elif defined(__riscv) && (__riscv_xlen == 64)
if (state->arch != KBOX_LOADER_ENTRY_ARCH_RISCV64)
__builtin_trap();
register uint64_t a0 __asm__("a0") = state->regs[0];
register uint64_t a1 __asm__("a1") = state->regs[1];
register uint64_t a2 __asm__("a2") = state->regs[2];
register uint64_t a3 __asm__("a3") = state->regs[3];
register uint64_t a4 __asm__("a4") = state->regs[4];
register uint64_t a5 __asm__("a5") = state->regs[5];
register uint64_t t0 __asm__("t0") = state->pc;
uint64_t sp = state->sp;

__asm__ volatile(
"mv sp, %0\n\t"
"jr t0\n\t"
:
: "r"(sp), "r"(a0), "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5), "r"(t0)
: "memory");
#else
(void) state;
__builtin_trap();
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6 changes: 3 additions & 3 deletions src/rewrite.c
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Expand Up @@ -608,10 +608,10 @@ static int encode_aarch64_virtual_procinfo_patch(
if (aarch64_prev_insn_syscall_nr(image, image_len, site_off, &nr) < 0)
return -1;

if (nr == (uint32_t) HOST_NRS_AARCH64.getpid ||
nr == (uint32_t) HOST_NRS_AARCH64.gettid) {
if (nr == (uint32_t) HOST_NRS_GENERIC.getpid ||
nr == (uint32_t) HOST_NRS_GENERIC.gettid) {
value = 1;
} else if (nr == (uint32_t) HOST_NRS_AARCH64.getppid) {
} else if (nr == (uint32_t) HOST_NRS_GENERIC.getppid) {
value = 0;
} else {
return -1;
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2 changes: 1 addition & 1 deletion src/seccomp-bpf.c
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Expand Up @@ -142,7 +142,7 @@ static const int deny_nrs[] = {
153, /* vhangup */
};

#elif defined(__aarch64__)
#elif defined(__aarch64__) || (defined(__riscv) && __riscv_xlen == 64)
static const int deny_nrs[] = {
/* Seccomp manipulation */
277, /* seccomp */
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2 changes: 2 additions & 0 deletions src/seccomp-defs.h
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Expand Up @@ -34,6 +34,8 @@
#define KBOX_AUDIT_ARCH_CURRENT 0xc000003eU
#elif defined(__aarch64__)
#define KBOX_AUDIT_ARCH_CURRENT 0xc00000b7U
#elif defined(__riscv) && __riscv_xlen == 64
#define KBOX_AUDIT_ARCH_CURRENT 0xc00000f3U
#else
#error "unsupported architecture"
#endif
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8 changes: 4 additions & 4 deletions src/seccomp-dispatch.c
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Expand Up @@ -33,28 +33,28 @@
#ifndef __NR_pidfd_open
#if defined(__x86_64__)
#define __NR_pidfd_open 434
#elif defined(__aarch64__)
#elif defined(__aarch64__) || (defined(__riscv) && (__riscv_xlen == 64))
#define __NR_pidfd_open 434
#endif
#endif
#ifndef __NR_pidfd_getfd
#if defined(__x86_64__)
#define __NR_pidfd_getfd 438
#elif defined(__aarch64__)
#elif defined(__aarch64__) || (defined(__riscv) && (__riscv_xlen == 64))
#define __NR_pidfd_getfd 438
#endif
#endif
#ifndef __NR_faccessat2
#if defined(__x86_64__)
#define __NR_faccessat2 439
#elif defined(__aarch64__)
#elif defined(__aarch64__) || (defined(__riscv) && (__riscv_xlen == 64))
#define __NR_faccessat2 439
#endif
#endif
#ifndef __NR_statx
#if defined(__x86_64__)
#define __NR_statx 332
#elif defined(__aarch64__)
#elif defined(__aarch64__) || (defined(__riscv) && (__riscv_xlen == 64))
#define __NR_statx 291
#endif
#endif
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4 changes: 2 additions & 2 deletions src/seccomp-supervisor.c
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Expand Up @@ -482,8 +482,8 @@ int kbox_run_supervisor(const struct kbox_sysnrs *sysnrs,
/* Architecture-specific host syscall numbers for the BPF filter. */
#if defined(__x86_64__)
const struct kbox_host_nrs *host_nrs = &HOST_NRS_X86_64;
#elif defined(__aarch64__)
const struct kbox_host_nrs *host_nrs = &HOST_NRS_AARCH64;
#elif defined(__aarch64__) || (defined(__riscv) && __riscv_xlen == 64)
const struct kbox_host_nrs *host_nrs = &HOST_NRS_GENERIC;
#else
#error "Unsupported architecture"
#endif
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2 changes: 1 addition & 1 deletion src/syscall-nr.c
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Expand Up @@ -353,7 +353,7 @@ const struct kbox_host_nrs HOST_NRS_X86_64 = {
.readlink = 89,
};

const struct kbox_host_nrs HOST_NRS_AARCH64 = {
const struct kbox_host_nrs HOST_NRS_GENERIC = {
.openat = 56,
.openat2 = 437,
.open = -1,
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2 changes: 1 addition & 1 deletion src/syscall-nr.h
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Expand Up @@ -220,7 +220,7 @@ struct kbox_host_nrs {
};

extern const struct kbox_host_nrs HOST_NRS_X86_64;
extern const struct kbox_host_nrs HOST_NRS_AARCH64;
extern const struct kbox_host_nrs HOST_NRS_GENERIC;

const struct kbox_sysnrs *detect_sysnrs(void);
const char *syscall_name_from_nr(const struct kbox_host_nrs *h, int nr);
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