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sysroot bypass hardcoded /etc/passwd and /etc/group intercepts #215

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Merged
jserv merged 1 commit into from
Jul 18, 2026
+32 −2
Merged

sysroot bypass hardcoded /etc/passwd and /etc/group intercepts #215

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2 changes: 2 additions & 0 deletions mk/tests.mk
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Original file line number Diff line number Diff line change
Expand Up @@ -257,6 +257,8 @@ test-sysroot-host-fallback: $(ELFUSE_BIN) $(BUILD_DIR)/test-sysroot-host-fallbac
printf 'host-final\n' > "$$mirror/final.txt"; \
printf 'host-loses\n' > "$$mirror/both.txt"; \
printf 'sysroot-wins\n' > "$$sysroot$$mirror/both.txt"; \
mkdir -p "$$sysroot/etc"; \
printf 'guest-passwd\n' > "$$sysroot/etc/passwd"; \
$(ELFUSE_BIN) --sysroot "$$sysroot" \
$(BUILD_DIR)/test-sysroot-host-fallback \
"$$hostdir" "$$mirror/final.txt" "$$mirror/both.txt"
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16 changes: 14 additions & 2 deletions src/syscall/path.c
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Expand Up @@ -56,9 +56,21 @@ bool path_might_use_open_intercept(const char *path)
return true;
if (path_prefix_match(path, SYSFS_CPU_PREFIX, sizeof(SYSFS_CPU_PREFIX) - 1))
return true;
if (!strcmp(path, "/etc/mtab") || !strcmp(path, "/etc/passwd") ||
!strcmp(path, "/etc/group"))
if (!strcmp(path, "/etc/mtab"))
return true;
if (!strcmp(path, "/etc/passwd") || !strcmp(path, "/etc/group")) {

@Max042004 Max042004 Jul 18, 2026

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path_might_use_open_intercept() only checks that a sysroot is configured
(proc_sysroot_snapshot()), not that the sysroot actually contains
/etc/passwd / /etc/group. When it doesn't, the request falls through to
proc_resolve_sysroot_path_flags()'s host-literal fallback, which resolves
the literal host path — on macOS that's the real /etc/passwd /
/etc/group on the machine running elfuse.

@doanbaotrung doanbaotrung Jul 18, 2026

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Fix Details

  1. Added Existence Checks: Updated path_might_use_open_intercept in path.c to perform a stat existence check on the guest sysroot-resolved files (${sysroot}/etc/passwd / ${sysroot}/etc/group). The synthetic intercept is bypassed only when the file is present in the sysroot.
  2. Updated Test Harness: Modified tests.mk to only stage passwd in the fallback test's guest sysroot, leaving group absent.
  3. Added Assertions: Updated test-sysroot-host-fallback.c to verify that the absent guest group file falls back to the synthetic group intercept instead of resolving to the host machine's /etc/group.

char sr[LINUX_PATH_MAX];
if (proc_sysroot_snapshot(sr, sizeof(sr))) {
char sysroot_file[LINUX_PATH_MAX];
if (snprintf(sysroot_file, sizeof(sysroot_file), "%s%s", sr, path) <
(int) sizeof(sysroot_file)) {
struct stat st;
if (stat(sysroot_file, &st) == 0)
return false;
}
}
return true;
}
if (!strcmp(path, "/var/run/utmp") || !strcmp(path, "/run/utmp"))
return true;

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16 changes: 16 additions & 0 deletions tests/test-sysroot-host-fallback.c
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Expand Up @@ -133,6 +133,22 @@ int main(int argc, char **argv)
else
PASS();

TEST("sysroot passwd file wins over synthetic intercept");
if (read_file_nul("/etc/passwd", buf, sizeof(buf)) <= 0)
FAIL("read /etc/passwd failed");
else if (strcmp(buf, "guest-passwd\n"))
FAIL("did not bypass /etc/passwd synthetic intercept");
else
PASS();

TEST("absent sysroot group file falls back to synthetic intercept");
if (read_file_nul("/etc/group", buf, sizeof(buf)) <= 0)
FAIL("read /etc/group failed");
else if (strncmp(buf, "root:x:0:\nstaff:x:20:\nuser:x:1000:\n", 32))
FAIL("did not fall back to synthetic group intercept");
else
PASS();

SUMMARY("test-sysroot-host-fallback");
return fails > 0 ? 1 : 0;
}
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