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Make GetenvBeforeMain work inside ifunc handler

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Depending on things such us strlen, which are ifunc-ed themselves is
not supported from inside ifunc handler.

Thus we implement strlen, memchr and memcmp in slow naive C++ just for
that place.
This commit is contained in:
Aliaksey Kandratsenka 2017-01-22 13:08:34 -08:00
parent a2550b6309
commit 7d49f015a0
1 changed files with 40 additions and 9 deletions
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49
src/base/sysinfo.cc
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@ -110,6 +110,40 @@
// Some non-trivial getenv-related functions.
// ----------------------------------------------------------------------
// we reimplement memcmp and friends to avoid depending on any glibc
// calls too early in the process lifetime. This allows us to use
// GetenvBeforeMain from inside ifunc handler
static int slow_memcmp(const void *_a, const void *_b, size_t n) {
const uint8_t *a = reinterpret_cast<const uint8_t *>(_a);
const uint8_t *b = reinterpret_cast<const uint8_t *>(_b);
while (n-- != 0) {
uint8_t ac = *a++;
uint8_t bc = *b++;
if (ac != bc) {
if (ac < bc) {
return -1;
}
return 1;
}
}
return 0;
}
static const char *slow_memchr(const char *s, int c, size_t n) {
uint8_t ch = static_cast<uint8_t>(c);
while (n--) {
if (*s++ == ch) {
return s - 1;
}
}
return 0;
}
static size_t slow_strlen(const char *s) {
const char *s2 = slow_memchr(s, '\0', static_cast<size_t>(-1));
return s2 - s;
}
// It's not safe to call getenv() in the malloc hooks, because they
// might be called extremely early, before libc is done setting up
// correctly. In particular, the thread library may not be done
@ -119,15 +153,12 @@
// /proc/self/environ has a limit of how much data it exports (around
// 8K), so it's not an ideal solution.
const char* GetenvBeforeMain(const char* name) {
const int namelen = slow_strlen(name);
#if defined(HAVE___ENVIRON) // if we have it, it's declared in unistd.h
if (__environ) { // can exist but be NULL, if statically linked
const int namelen = strlen(name);
for (char** p = __environ; *p; p++) {
if (strlen(*p) < namelen) {
continue;
}
if (!memcmp(*p, name, namelen) && (*p)[namelen] == '=') // it's a match
return *p + namelen+1; // point after =
if (!slow_memcmp(*p, name, namelen) && (*p)[namelen] == '=')
return *p + namelen+1;
}
return NULL;
}
@ -155,14 +186,14 @@ const char* GetenvBeforeMain(const char* name) {
}
safeclose(fd);
}
const int namelen = strlen(name);
const char* p = envbuf;
while (*p != '\0') { // will happen at the \0\0 that terminates the buffer
// proc file has the format NAME=value\0NAME=value\0NAME=value\0...
const char* endp = (char*)memchr(p, '\0', sizeof(envbuf) - (p - envbuf));
const char* endp = (char*)slow_memchr(p, '\0',
sizeof(envbuf) - (p - envbuf));
if (endp == NULL) // this entry isn't NUL terminated
return NULL;
else if (!memcmp(p, name, namelen) && p[namelen] == '=') // it's a match
else if (!slow_memcmp(p, name, namelen) && p[namelen] == '=') // it's a match
return p + namelen+1; // point after =
p = endp + 1;
}