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src: use _init as the start of large pages #31947

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src: use _init as the start of large pages #31947

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51 changes: 42 additions & 9 deletions src/large_pages/node_large_page.cc
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Expand Up @@ -70,8 +70,10 @@
// If successful copy the code there and unmap the original region.

#if defined(__linux__)
#include <dlfcn.h> // For retrieving _init at runtime
extern "C" {
extern char __executable_start;
extern char __start_lpstub;
} // extern "C"
#endif // defined(__linux__)

Expand Down Expand Up @@ -106,6 +108,20 @@ inline uintptr_t hugepage_align_down(uintptr_t addr) {
return ((addr) & ~((hps) - 1));
}

#if defined(__linux__)
inline uintptr_t RetrieveInitOffset() {
uintptr_t init_offset = 0;
void* dlhandle = dlopen(nullptr, RTLD_NOW | RTLD_NOLOAD);
if (dlhandle != nullptr) {
init_offset = reinterpret_cast<uintptr_t>(dlsym(dlhandle, "_init"));
if (dlclose(dlhandle) != 0) {
PrintWarning("Failed to dlclose() self after retrieving _init");
}
}
return init_offset;
}
#endif

// The format of the maps file is the following
// address perms offset dev inode pathname
// 00400000-00452000 r-xp 00000000 08:02 173521 /usr/bin/dbus-daemon
Expand All @@ -121,6 +137,7 @@ struct text_region FindNodeTextRegion() {
std::string dev;
char dash;
uintptr_t start, end, offset, inode;
uintptr_t init_offset = RetrieveInitOffset();

ifs.open("/proc/self/maps");
if (!ifs) {
Expand All @@ -147,15 +164,31 @@ struct text_region FindNodeTextRegion() {
if (start != reinterpret_cast<uintptr_t>(&__executable_start))
continue;

// The next line is our .text section.
if (!std::getline(ifs, map_line))
break;
// On Ubuntu 18.04 the binary gets loaded into a single mapping. So, before
// we make the assumption that the next mapping contains the .text section
// we check if this mapping contains the symbol `_init` -- which, on
// Ubuntu 18.04 it does. If so, we calculate `start` and `end` from this
// mapping and take into account that we must exclude the section `lpstub`
// from the returned range, because `lpstub` contains the code responsible
// for re-mapping the .text section, and we don't want it re-mapping itself
// as it's doing that, because that will cause the process to crash.
if (init_offset != 0 && init_offset >= start && init_offset < end) {
uintptr_t lpstub_start = reinterpret_cast<uintptr_t>(&__start_lpstub);
if (lpstub_start > start && lpstub_start <= end) {
end = lpstub_start;
}
start = init_offset;
} else {
// The next line is our .text section.
if (!std::getline(ifs, map_line))
break;

iss = std::istringstream(map_line);
iss >> std::hex >> start;
iss >> dash;
iss >> std::hex >> end;
iss >> permission;
iss = std::istringstream(map_line);
iss >> std::hex >> start;
iss >> dash;
iss >> std::hex >> end;
iss >> permission;
}

if (permission != "r-xp")
break;
Expand Down Expand Up @@ -318,7 +351,7 @@ static bool IsSuperPagesEnabled() {
// d. If successful copy the code there and unmap the original region
int
#if !defined(__APPLE__)
__attribute__((__section__(".lpstub")))
__attribute__((__section__("lpstub")))
#else
__attribute__((__section__("__TEXT,__lpstub")))
#endif
Expand Down